This is the sixteenth post on Christian ethics in my ongoing series, theology in bullet points. The first unit in this series had to do with God and Creation (book here), and the second unit was on Christology and Atonement.
We are now in the third and final unit: The Holy Spirit and the Church. The first set of posts in this final unit was on the Holy Spirit. The second set was on the Church. The third set was on sacraments. This final section is on Christian ethics.
Sex should be kept within a marriage relationship.
1. When Jesus presented a model of marriage in Matthew 19, he looked to Genesis 2 as the model: "Have you not read that the one who made them at the beginning 'made them male and female,' and said, 'For this reason a man shall leave his father and mother and be joined to his wife, and the two shall become one flesh'? So they are no longer two, but one flesh. Therefore what God has joined together, let no one separate" (19:4-6).
Becoming "one flesh" here is bigger than sex, but it certainly includes it. Paul prohibits visiting a prostitute by reminding the Corinthians that such an individual becomes one flesh with that prostitute (6:16). Paul is not in any way saying such a person is marrying the prostitute. But he is indicating that sex is an act that has dramatic spiritual implications and that it should only take place within marriage. 
Today we focus especially on the relational aspects to sex, the way it involves a special bond between two people. In the biblical world, the social implications and the purity implications would have been even more important. Sex was understood to be an act of particular vulnerability for defilement. And unless it took place within marriage, it threatened the social stability of society.
2. There are not many explicit statements in the Bible insisting on sex only in marriage because it was such a strong assumption of the biblical world. It did not need to be said. Women especially were expected to be virgins when they marry. It was not necessary to argue for it.
The older word fornication was often taken to refer to sex before marriage, but the underlying word porneia had a very broad reference to sexual immorality in general. When 1 Thessalonians 4:3 says to "abstain from fornication," it was not likely singling out pre-marital sex from other forms of sex outside marriage, but it was referring to a variety of improper sexual activity, perhaps including adultery.  Paul says not to exploit a brother in this area, quite possibly a comment on adultery (4:6).
3. The Old Testament clearly expected a woman to be a virgin when she married. In Deuteronomy 22, a woman who is not found to be a virgin on her wedding night was to be stoned (22:20-21). If a man sleeps with an engaged woman in the country, he is to be stoned but she is spared on the assumption that she screamed but no one was there to hear (22:25-27). If the same takes place in the city both are to be stoned on the assumption that she could have screamed but did not (22:23-24).
If a man sleeps with a woman who is not engaged, he has to marry her (Deut. 22:28-29). Exodus' version of this law also allows that the father may not want the woman to marry this man (Exod. 22:16-17). Both passages reflect the fact that a woman was expected to be a virgin at the time of marriage.
4. 1 Corinthians 7 gives perhaps the clearest instruction on the subject: "To the unmarried and the widows I say that it is well for them to remain unmarried as I am. But if they are not practicing self-control, they should marry. For it is better to marry than to be aflame with passion" (78-9). In other words, if you cannot control your sexual urges, it is better to get married than to be having indiscriminate sex outside of marriage. 
This is a very practical approach. It does not give us the lofty theology of man and woman becoming one flesh. It simply states that people should get married if they can't control themselves sexually.
5. We shouldn't be surprised that much of the biblical text focuses on the woman. The woman was seen in ancient patriarchal cultures as a point of vulnerability for shame to one's family. But most avenues for a man to have sex outside of marriage were also closed. He could not have sex with someone else's wife or concubine--that was adultery or porneia. He could not have sex with a virgin, for that wronged her and her family. Israelite men and women were not allowed to be prostitutes (Lev. 19:29; Deut. 23:17).
In short, the biblical text insists that women can only have sex within marriage, and the avenues for a man to have sex outside of marriage were prohibited.
6. In current Western culture, defilement has been transformed into sexually transmitted diseases. Social consequence has largely been translated into the difficulties and disadvantages of single parenting. These are of course real possible consequences of sexual activity outside of exclusive sexual relationships that involve commitment. Any teenager or twenty something whose attitude to sex is casual takes the risk of significantly disadvantaging their lives in these ways.
But there is also a potential cheapening of life. One lives for selfish pleasure, the baser and less noble of human impulses. If love of others is virtue, indiscriminate sexuality is on the love of self side of the equation. Sex can be a high expression of love, or it can be an act of pure self-pleasuring at the expense of others. The latter is incompatible with the values of Christ.
The more one values marriage, the more one will restrict sex to the marriage relationship. And the more one has the values of Christ, the more one will view males and females as equal partners in sexual relationships. Paul shockingly declares these values of mutuality in 1 Corinthians 7:1-5 where he urges that husbands and wives should have sex regularly so that they are not tempted to find sex elsewhere. He shockingly suggests that the wife has claim on her husband's body, just as he has claim on hers.
7. In the end, there can be no question where the assumption of Scripture lies. The Bible assumes throughout that sex is to be practiced within marriage and not outside of it.
Next Sunday: ET17: The Bible prohibits homosexual sex and several other types of sexual act.
 We can extend the principle and say that a man in the Old Testament with many wives became one flesh with all of his wives.
 One might look to Leviticus 18 for a general sense of the kinds of activities to which porneia refers.
 Paul's preference for celibacy here probably had a lot to do with an early expectation that Christ would return rather quickly to earth (e.g., 1 Cor. 7:29-31). 1 Timothy would later encourage marriage as the norm (1 Tim. 4:3; 5:14).