According to the apostle Paul, even when someone has sexual relations with a prostitute, he has "become one flesh" (v. 16) with her. And note that the reference to "one flesh" in verse 16 is quoted directly from Genesis2:24. (And, of course, Genesis 2:24 is speaking primarily about Adam andEve, and that even before sin!)
Then in verse 17, Paul shows that a believer's spiritual union ("joined to the Lord is one spirit with Him"--NKJV) with the Lord is a contrast to the physical (ie., sexual) union with a prostitute. This means that sexual union (ie., intercourse) between a married couple is supposed to represent the spiritual union that each one is to have with the Lord. Therefore, under the Biblical conditions, the sexual act is a symbol of something spiritual. If one has sex with a prostitute, or anyone other than his/her married partner, then that act illustrates a breakage in the spiritual union with the Lord--as well as the spouse.
The spiritual dimension of sex (explained above) is the reason, then, that sex outside of marriage is called "sexual immorality" (v. 18, NKJV;"fornication", as in KJV, is translated from the Greek word from which we get the word "pornography").
Verse 19 explains further that one's body is actually the temple of the Holy Spirit--which adds to the earlier evidence that what we do with our physical body has spiritual implications. Thus, verse 20 declares that we ought to glorify God in our body (by what we do with it).
In these verses, the apostle Paul cautions that divorce is not God's ideal (v. 10). However, if it does occur, then the spouses are not free to remarry. [Jesus Himself said the same thing in different language inMark 10:1-12; and in Matthew 5:32, Jesus makes fornication the only valid reason for divorce and remarriage.]
Then in discussing the relationship between a believing spouse and an unbelieving spouse, Paul stated that if the unbelieving spouse wants a divorce, then the believing spouse should give it to him/her. Otherwise, however, the believing spouse should remain in that marriage. Notice inverse 15 that Paul says the believing spouse who is divorced by his/her partner is then "not under bondage" (NKJV). That "bondage" concept Paul also used in Romans 7:2 with regard to marriage and when a believer is free to remarry.
After considerable discussion about virgins and widows in the church(verses 25-38), the apostle Paul speaks about the fact that a spouse is"bound" by law to his/her marriage partner until he dies.(1)Then he/she is free to remarry, as long as it is to another believer, of course. The immediate context of the verses since verse 16 means that verse39 is speaking about both spouses who are believers.
Note that the apostle Paul states in verse 15 that if an unbelieving spouse divorces a believing partner, the latter is "not bound"--in other words, he/she is free to remarry. This freedom to remarry is not dependent upon whether the unbelieving spouse who divorced his/her partner remarries or dies. However, in verse 39, Paul emphatically declares that a divorced believer is not free to remarry unless the other former partner dies--or we should rightfully add (on the basis of Jesus' statement--see above), unless he/she commits fornication by sleeping with someone else, or remarries. By the way, this different counsel in verse 39 adds proof to the idea that here Paul is talking about two believers.
Why does Paul give such different instruction to believers who are divorced by unbelievers from what he gives to two believers who become divorced?Only one answer seems evident in Scripture--especially since that answer is in the close context of this same letter to the Corinthian church: Since he has just told us (in 6:16-20--see above) that the sexual act was designed to represent a spiritual unity between two people which mirrors the unityGod wants with each of us, then God must view the sexual act between a believer and an unbeliever only as a physical union; after all, it takes two to produce a union of any kind in marriage, and since the unbelieving spouse is not connected spiritually to the Lord, then sex in that marriage cannot represent that spiritual union with Him. But with two believing marriage partners, that spiritual union is present. As to the objection that one can be in the church and not be spiritually connected to God, remember that when His people in the Old Testament were behaving badly, He still referred to them as His people. Therefore, God still views two church members as spiritually married to each other despite a human legal divorce because their spiritual union has not been broken (assuming neither one has committed fornication, remarried, or died), even though their physical union has been broken.(2)