I have some impressions of what I might conclude if I did a more thorough study of Paul and sexual sin, but I've never actually done it. I thought I might blog in brief on the subject.
The point of real exploration is not so much about what acts Paul considered to be inappropriate as why he considered them to be so. To the extent to which such perspectives are unexpressed in the text--indeed, Paul himself may not have been completely aware of the forces on his thinking--to that extent this is an exercise about which our conclusions will remain somewhat uncertain even after we have explored all the evidence we have.
Additionally, it is not clear to me that it is Paul's underlying thinking that gives us the meaning of these texts as Scripture. That meaning seems to me something the church discerns from the Holy Spirit. Nevertheless, the more we know, the freer we are to make good choices.
I might begin with 1 Thessalonians 4:3: "This is the will of God, your sanctification, for you to abstain from porneia." The crucial question here is what exactly Paul means by "porneia." The meaning of porneia for Paul was obvious to him. Perhaps it was obvious to the Thessalonians as well. I have suggested elsewhere that Leviticus 18 perhaps gives us the best summary of what Paul included in this word.
In 1 Thessalonians, Paul does give us the flavor of what porneia is. He equates abstaining from porneia to "each of you possessing your own vessel in holiness and honor" (4:4). He equates this comment with "not possessing it with the passion of desire like the Gentiles who do not know God also do" (4:5) and "not to offend and defraud your brother in this matter." The words used in this last statement have overtones of "taking over" (hyperbaino) and "taking more than your due" (pleonekteo). In short, I think Paul must primarily have either adultery or sleeping with someone promised to another in view.
By the way, this word does not refer exclusively or even primarily to pre-marital sex, as the KJV translation "fornication" is sometimes taken. It probably includes pre-marital sex, as one possible interpretation of this passage indicates.
I believe the following statement to be true: There is no act of sex outside marriage that Paul does not consider to be sin. But what is interesting is that Paul, and all the other biblical writers, do not argue that such sex is sin because it is outside marriage. In other words, biblical authors argue that various kinds of sex outside marriage are sins, but they never argue that they are sins because they take place outside marriage. They do not argue that a person can only have sex within marriage and that any sex a person has outside marriage is a sin, although this is what their position amounts to. Rather, they argue that specific types of sex are sins and they do this with nearly every specific type of sex other than the act of sex within marriage. This is a major difference between Paul's sexual paradigm and the current evangelical one.
Let us go through the various sexual sins that the Bible discusses to demonstrate this claim:
1. Sex with a Prostitute (1 Corinthians 6)
I start with this category because Paul's comments on this issue are instructive. Paul makes it clear to the Corinthians that they should not join their "members" to a prostitute. The reason is that they defile the body both of Christ and of themselves. Here we note that Paul does not say that the act of sex defiles because it is sex outside marriage. It defiles because of who you are becoming "one flesh" with.
Note the distinction: Paul does not argue that you shouldn't have sex with a prostitute because you have become one flesh with your wife (and Paul's arguments are male-oriented). This would be the way a contemporary evangelical would argue the prohibition. Rather, Paul argues that you should not have sex with a prostitute because you become one flesh with her! (and she's "icky")
Adultery is consistently understood to be a sin in the Bible. However, the Bible never says that it is sin because any sex outside marriage is a sin. In the categories of both Leviticus and the New Testament world, adultery shames the husband of the wife with whom a man sleeps. Adultery is thus argued to be wrong because it is shameful and has incredible social consequences. The woman seems to get a raw deal here--the ancient world didn't speak of committing adultery against a wife. Adultery seems to have always been formulated as an offence against a man.
In Leviticus 18, almost every act prohibited follows the pattern Do not uncover the nakedness of x (a man) by sleeping with y (a woman who stands in some honor-shame relationship with x). The NIV, as many translations, has eliminated the honor-shame language of nakedness. Again, in none of these places does the text locate the problem as one of sleeping outside your marriage. The problem comes from who you are sleeping with and, thus, from the fact that you are shaming some man.
And here we remind ourselves again that God has moved His church to a more perfect understanding of so many of these things. So I can reiterate that I think the current evangelical way of arguing these things is possibly the way God wants us to argue them. But in the ancient world, adultery was understood as the shaming of a man by lying with his wife. In that sense, the OT would not have defined what Judah does with Tamar in Genesis as an act of adultery. Tamar is of course his daughter-in-law, and she dresses as a prostitute because Judah has not had his youngest son go in to her to raise seed for her dead husband (long story). She gets pregnant from her father-in-law. The Genesis story treats her as righteous in her actions.
What is interesting is that Judah is not condemned in the story for visiting a prostitute, which would not have been considered adultery in his day because he was shaming no man by sleeping with one. We remind ourselves that Paul prohibits sleeping with a prostitute in the NT, but such a prohibition was not on the books (there were no books) at the time of Judah. But this act would not have been classified as adultery even in Paul's day. Paul considered it sinful and defiling, but would not have called it adultery.
Jesus' words in Matthew 5 and 19 are thus shocking, because he suggests that a man commits adultery against himself when he divorces his wife. Normally, divorcing your wife would not be considered adultery. But as usual, Jesus is being provocative. A man who divorces his wife, in effect forcing her to marry another man (how else would she survive?), causes her to commit adultery against himself. Jesus' words set in motion a process that reaches full bloom with us, for we rightly believe that a man commits adultery against his wife when he sleeps with anyone but her. But I do not think you will not find this concept clearly articulated anywhere in the Bible.
A previous string of entries have made it clear that Paul considered the act of sex with someone of the same gender to be shameful and defiling. What he nowhere argues, however, is that the act is defiling because it is outside marriage.
I believe it was the assumption of Paul's world that a woman should be a virgin when she marries. He never argues this, but he assumes it throughout 1 Corinthians 7. It is so deep an assumption of Paul's world that he doesn't even think to argue it. As far as a man is concerned, what avenue of sex is there outside marriage that Paul would allow? He prohibits sex with a prostitute, sex with someone's wife, homosexual sex, etc... What is left?
Paul implies that pre-marital sex is inappropriate in two ways. First, he says it is better to marry than to burn (with passion). This statement appears in 1 Corinthians at a place where Paul is arguing that celibacy is not for every man. He argues that if you cannot control your passions, you should marry. Second, this argument is similar to what Paul has said in 1 Thessalonians 4: don't possess your vessel with the passion of desire. Pre-marital sex implies an inability to control your "passion" and "burning."
In Paul's day it "defrauded" the father and perhaps appointed spouse as well. It also cheapened the virgin, making her an object of shame. Couples who cannot wait for marriage today often experience a similar shame and compromising of value in the church today as well--which is in continuity with the biblical world.
I add this section at the request of a colleague. As far as I can tell, there is no biblical passage that addresses this topic. Some use the passage about Onan in Genesis, but it has nothing to do with it. Onan was required to raise seed for his dead brother and he didn't. He was more than happy to have sex with Tamar, but not willing to climax with her so that she would get pregnant, even though that was his duty. He didn't want the rights of his own first born to be assigned to his dead brother. In short, this passage is about a whole lot of things that aren't masturbation.
I doubt very seriously that any biblical author would have considered maturbation under the heading of "sex," so in some ways it doesn't fit in this discussion. Paul might address it under the "passion of desire" clause.
So under the "I too have the Spirit of God" category, let me suggest a few things:
1. What makes masturbation dangerous and sinful for a Christian is what you are thinking. Are you lusting after someone? Then it comes under the Matthew 5 heading--don't do in your mind what you shouldn't do in real life. Ask yourself, "How would this girl react if she knew I was thinking this?" Lust cheapens the person you are lusting after. I have known guys who have claimed not to lust while doing this, and I believe it is possible. God is ultimately the judge of intentions (1 Cor. 4). Sometimes we feel like we have to be able to catch those who are lying or rationalizing, but no one ever fools God. He is the judge.
2. "Moderation in all things." That's the Golden Mean. A man's body has a cycle like a woman's--it just isn't tied to a regular sequence of days. Let's say about every three weeks or so the male body gets to a certain point where thinking is not involved.
Well, enough on that... Run away...
So we can add these individual prohibitions and come to the conclusion that "sex is only appropriate within marriage." That is the current evangelical position. But notice that Paul did not formulate it the way we do. Paul prohibited all the various avenues of sex outside marriage. Our formula is much simpler and reaches the same conclusion: Christians should only have sex within marriage. I think God has helped the church progress in this formulation, for it treats the woman equally to the man in every way, and I'm convinced that this is God's way.
To wend my usual wares, I make my usual observations:
1. To read the Bible the way we know we need to read it, we must often go beyond what it actually says and means. The Bible alone could be used to support a world where a man can sleep with someone who isn't his wife without committing adultery.
2. Christians extend the biblical meanings in this way all the time. Although there is no passage anywhere in the Bible that says "Do not have sex with anyone but your spouse" or "It is a sin to have sex with anyone but your spouse," most Christians would insist it does say this. This is the implication of the biblical teaching, but the Bible nowhere says this.
3. It is the "Bible-as-churched" that is authoritative for Christians, not simply the original meaning. It is the meaning these words take on when we bring a "Christian dictionary" to bear on them, and this dictionary is the dictionary provided by the communion of saints through the ages, in whom the Spirit of God dwells. Such a dictionary is usually in continuity with the original meaning, but it is usually a little different and often quite different.