A: This is a very observant question, as there are a number of passages throughout the Scriptures that explicitly condemn "sexual immorality" without a description of exactly what this means! As a result, you will find a great range of answers to a question like this, especially amongst different denominations.
Unfortunately, many of the disagreements in this particular area are not based on careful examination or exegesis of the Scriptures, but are really a product of personal preferences and cultural mores. While some of these "definitions" are probably better than others, it is always dangerous to allow our culture to interpret the Scriptures instead of the Scriptures interpreting our culture (Romans 12:1-2) -- especially when living in a "sex crazed" culture such as ours!
Sex is a touchy topic, and unfortunately one that is not discussed much within the Church. That probably seems strange... but I mean exactly that: the Church should talk more about sex than it does, because when the Church fails to allow the light of the revelation of God to shine in different areas, we should not be surprised to find that these same areas drift into darkness.
When a society moves into moral relativism (a direction in which the United States is moving further and further with each passing moment), sin abounds. We are not the first to "experiment" with doing what is right in our own eyes. Just take some time to read through the book of Judges sometime to see what can happen when everyone does what is right in their own eyes ... it isn't pretty!
Instead of walking in the darkness, Jesus calls us to follow Him and walk in the light (John 8:12). The Church has been entrusted with the revelation of God (Jude 1:3) and is supposed to serve as the "pillar and support of the truth" so that believers may know how to conduct themselves, both in the Church and in the world (1 Timothy 3:15; 2 Timothy 3:16-17; Colossians 4:5). Therefore, when the Church stops teaching on sex, this important area of human experience drifts into darkness. We sometimes think we are being "proper" by failing to talk about such things, but in reality the Scriptures have a lot to say about sex! To fail to discuss matters relating to human sexuality is to fail to teach the full counsel of God.
Some think it is good and right to ignore topics relating to human sexuality because they believe that sex itself is inherently sinful. Some have even based this "teaching" on passages such as the following:
Behold, I was brought forth in iniquity, And in sin my mother conceived me. (Psalm 51:5, NASB)
However, this is not the best way to understand this passage. Instead, David is pointing to his depraved and fallen state, expressing that he himself was "in sin" from the moment of conception, not that his conception was a result of a sinful sexual act by his parents. The whole Psalm is about David's personal guilt and the interpretation that this is describing "sex" as sinful is simply forced. The NET Bible has a good note on this passage which is worth quoting in full:
Heb "Look, in wrongdoing I was brought forth, and in sin my mother conceived me." The prefixed verbal form in the second line is probably a preterite (without vav [w] consecutive), stating a simple historical fact. The psalmist is not suggesting that he was conceived through an inappropriate sexual relationship (although the verse has sometimes been understood to mean that, or even that all sexual relationships are sinful). The psalmist's point is that he has been a sinner from the very moment his personal existence began. By going back beyond the time of birth to the moment of conception, the psalmist makes his point more emphatically in the second line than in the first.
In fact, to interpret this passage to mean that sexual intercourse is inherently sinful causes severe problems with the rest of the Scriptures. God created humanity in His image and commanded that they be fruitful and multiply, which is essentially a command to have sexual relations since that is how human beings reproduce! God does not command us to sin.
Similarly, in the New Testament, the Scriptures teach that it is not beneficial for married couples to abstain from sexual relations for a long period of time:
Now concerning the things about which you wrote, it is good for a man not to touch a woman. But because of immoralities, each man is to have his own wife, and each woman is to have her own husband. The husband must fulfill his duty to his wife, and likewise also the wife to her husband. The wife does not have authority over her own body, but the husband does; and likewise also the husband does not have authority over his own body, but the wife does. Stop depriving one another, except by agreement for a time, so that you may devote yourselves to prayer, and come together again so that Satan will not tempt you because of your lack of self-control. (1 Corinthians 7:1-5, NASB)
Notice two things about this passage: First, Paul writes under the inspiration of the Holy Spirit that "because of immoralities, each man is to have his own wife, and each woman is to have her own husband." The solution to "sexual immorality" is not to abstain from sexual relations, but to have a healthy sexual relationship with your spouse. In this passage Paul is explicitly teaching that the best antidote to sexual immorality is a healthy dose of moral sexual behavior!
Secondly, Paul teaches that to abstain from healthy sexual behavior for too long is not a good thing, because this creates opportunity for Satan to tempt us through our lack of self-control.
However, this still doesn't fully answer the question of what is "sexually moral" behavior. Paul seems to be operating on an assumption that his readers understand what is appropriate sexual behavior and what is not and he is not interested in giving a list of "approved sexual behaviors." In fact, the Scriptures consistently take this tact -- never resorting to explicit uses of imagery or language that will arouse the various lusts and impulses of the flesh. The Scriptures are not meant to tempt us, but to train us in righteousness and holiness and to point us to Christ.
As a result, we can have some difficulties if we are expecting that the Lord will simply provide a list of "approved" sexual acts. This would be borderline pornographic, which is why I believe we do not see such a list.
Some groups have made the error of attempting to validate certain sexual behaviors through finding "examples" of such behavior that are not explicitly condemned as sinful in the Scriptures. An example of this is the teaching that polygamy is a valid practice based on the actions of the Patriarchs (e.g. Jacob). This is a serious error, since the Bible records faithfully the sinful acts of many people. It is sobering to realize that there is only one person in all of Scripture who was without sin: Jesus. Everyone else -- every "hero" of the faith -- is terribly sinful compared to a righteous God. To interpret what happened in the lives of sinful people, especially in narrative accounts, as what ought to happen is a mistake.
To make better sense of all of this, it is good to turn our attention to the heart of the question above: How are we supposed to know what "sexual immorality" is? The Scriptures define both negative ("Don't") and positive ("Do") commands and affirmations. To best understand the negative (abstain from sexual immorality) it is helpful to understand the positive; namely, what is the scriptural definition of "sexual morality"? If we know what is "sexually moral" then we know that anything outside of this is "sexually immoral."
The definition of sexual morality is given very early in the Scriptures:
For this reason a man shall leave his father and his mother, and be joined to his wife; and they shall become one flesh. (Genesis 2:24, NASB)
As a part of God's very good original creation, and in order for the Man (both male and female, see Genesis 1:27) to fulfill his purpose multiplying over the face of the earth and of ruling over the earth and subduing it, there was given the institution of "marriage between a man and a woman."
One man, one woman, for the purpose of fulfilling God's plan.
Have you ever thought about the fact that the sexual relationship between a man and a woman is the most powerful creative act that human beings can engage in? The sexual act is the only way that the most amazing creation of our God (another human being) can be formed! No wonder this powerful act can be so dangerous when corrupted from its original design and intention!
If we understand this passage to be the definition of "sexual morality" -- i.e. the moral place for sexual relationships is within the monogamous confines of a relationship between one man and one woman -- then we can see that the Scriptures do not need to condemn polygamy when it is described later in Genesis (and throughout the Scriptures) because it has already defined sexually moral behavior and polygamy falls outside of that scope!
Likewise, if we understand that this is how the Scriptures define the moral sexual relationship, now we can understand the seeming "silence" of the Scriptures on issues like "premarital sex." I've heard some claim that the Bible doesn't ever condemn premarital sex, so therefore, sex before marriage isn't part of "sexual immorality."
However, can you find a Scripture that defines a marriage ceremony like what we practice today? I bet you can't....
How does the Bible define "marriage"?
There are a number of passages that could be quoted, but perhaps the clearest is found in Deuteronomy 21:13b -- ...and after that you may go in to her and be her husband and she shall be your wife. (Deuteronomy 21:13, NASB)
This passage explicitly states that the consummation of the physical act of intercourse is what joins a man and a woman together as husband and wife! So why doesn't the Bible discuss "premarital sex"? Because there is no such thing!
The moment the physical act is consummated, the two are joined together in the eyes of God until the death of one or both partners. God is serious about sex!
Re-read Genesis 2:24 -- For this reason a man shall leave his father and his mother, and be joined to his wife; and they shall become one flesh.
There is no need for an "officiant" and witnesses and a party or any of that stuff ... once they are "joined" they are one. This doesn't diminish the importance of a ceremony and the rest, but it does point to the fact that marriage is not exactly the same in the eyes of God as it is in the eyes of our society. The ceremony is supposed to point to the commitment to stay faithful to each other. However, even our laws represent an understanding that the "marriage" is only valid if it has been consummated.
If this positive understanding of "sexual morality" is right, this helps us to understand the negative commands even more clearly as there are some things which God commands that be refrained from even within the confines of marriage. The Law recorded several prohibitions regarding sexual conduct that applied to the nation of Israel in particular. The Church is no longer under the Law, but these principles are still valuable for us to understand (2 Timothy 3:16-17). Space does not permit us to examine each of these in great detail, but it is worth a prayerful read through the Pentateuch (i.e. the first five books of the Hebrew Bible: Genesis, Exodus, Leviticus, Numbers and Deuteronomy) to see how God discusses sexual relations between husbands and wives.
There is one final aspect of Scriptural teaching in the New Testament that must be understood as it relates to "sexual morality." Jesus defined the 7th Commandment to be more than simply the outward act of marital unfaithfulness by extending it even to the thoughts and intentions of the heart.
You have heard that it was said, 'YOU SHALL NOT COMMIT ADULTERY'; but I say to you that everyone who looks at a woman with lust for her has already committed adultery with her in his heart. (Matthew 5:27-28, NASB)
Simply put, this means that God intends for 100% of all sexual fulfillment to be had within the confines of the marriage relationship. This includes physical, emotional, and mental fulfillment. Since this is the case, it makes even more sense why God would inspire Paul to write what was already quoted above from his letter to the Corinthians:
But because of immoralities, each man is to have his own wife, and each woman is to have her own husband. The husband must fulfill his duty to his wife, and likewise also the wife to her husband. The wife does not have authority over her own body, but the husband does; and likewise also the husband does not have authority over his own body, but the wife does. Stop depriving one another, except by agreement for a time, so that you may devote yourselves to prayer, and come together again so that Satan will not tempt you because of your lack of self-control. (1 Corinthians 7:2-5, NASB)
Sex is not sinful by itself, and it is not wrong to be enjoyed. In fact, there is an entire book devoted to the joy of sexual morality in the Scriptures: Song of Solomon. As a result of the overtly sexual nature of this book, it has been debated as to whether or not this belongs in the Scriptures (since it seems to clearly be talking about sex), and once it was decided that it would be inappropriate to remove this book from the canon, various "spiritual" interpretations were suggested (especially by Christian believers) that make the book about something other than sex (most often, as an allegory for Christ and the Church).
I am in agreement with John MacArthur about the proper interpretation and handling of Song of Solomon. MacArthur wrote a good 5-part blog post (the first part is available here, and the interested reader can find the links to the next parts at the bottom of that page) in response to the opposite error of making the Song a "how to manual" with commands to engage in certain explicit acts by another famous pastor, which is worth reading in its entirety.
The major point is this: the Scriptures define "sexual morality" as the complete (100%) fulfillment of all sexual activity (whether physical, emotional, or mental) within the confines of a marriage between one man and one woman. Other scriptural prohibitions regarding sexual activity should therefore be understood as applying to the marriage relationship and further defining and restraining sexual fulfillment. These restrictions are not meant to prohibit sexual activity, but instead to properly focus all sexual activity in a way that is God-honoring and holy. Failure to operate within these parameters as created by God is to be "sexually immoral."
Sex is a gift from God, and when understood and used properly it is a beautiful thing. As the most powerful creative act that human beings can engage in, it is also extremely dangerous when perverted and used outside of the parameters of "sexual morality." Hopefully, this helps shed some light on this topic so that you may walk in holiness and purity before your God!
1 Thessalonians 4:1-8
(A follow up question was asked in the comments and addressed in Part 2)