Kay

To me, context here is very important. In the verses just preceding 1 Cor. 7:4-5 it says,

“You know that your bodies are parts of the body of Christ. Shall I take a part of Christ’s body
and make it part of the body of a prostitute? Impossible! Or perhaps you don’t know that the
man who joins his body to a prostitute becomes physically one with her? The scripture says
quite plainly, “The two will become one body.” But he who joins himself to the Lord becomes spiritually one with him. Avoid immorality. Any other sin a man commits does not affect his
body; but the man who is guilty of sexual immorality sins against his own body. Don’t you know that your body is the temple of the Holy Spirit, who lives in you and who was given to you by God? You do not belong to yourselves but to God; he bought you for a price. So use your bodies for God’s glory.” 1 Cor. 6:15-20

Since chapter/verse divisions were not originally part of this letter and considering that the sex act is THE defining difference between marriage and all other relationships, I see verses 15-20 as related to 7:4-5 (and all of ch. 7 for that matter). Every Christian – our body, soul and spirit – no longer belong to ourselves, but to God. “So use your bodies for God’s glory.” 1 Cor. 6:20

Now, I also notice a great similarity between the verses above and the beginning of Ephesians 5:
“Since you are God’s dear children, you must try to be like him. Your life must be controlled by love, just as Christ loved us and gave his life for us as a sweet-smelling offering and sacrifice that pleases God. Since you are God’s people, it is not right that any matters of sexual immorality or indecency or greed should even be mentioned among you. Nor is it fitting for you to use language which is obscene, profane, or vulgar. Rather you should give thanks to God. You may be sure that no one who is immoral, indecent, or greedy (for greed is a form of idolatry) will ever receive a share in the Kingdom of Christ and of God. Do not let anyone deceive you with foolish words; it is because of these very things that God’s anger will come upon those who do not obey him. So have nothing at all to do with such people. You yourselves used to be in the darkness, but since you have become the Lord’s people, you are in the light. So you must live like people who belong to the light, ” vs. 1-8.

and these verses at the end of ch. 5:
“In this same way, husbands ought to love their wives as their own bodies. He who loves his wife loves himself. After all, no one ever hated his own body, but he feeds and cares for it, just as Christ does the church— for we are members of his body. “For this reason a man will leave his father and mother and be united to his wife, and the two will become one flesh.”This is a profound mystery—but I am talking about Christ and the church. However, each one of you also must love his wife as he loves himself, and the wife must respect her husband.”

We are all parts of Christ’s Body. One part cannot use another part at will.

“Or perhaps you don’t know that the man who joins his body to a prostitute becomes physically one with her? The scripture says quite plainly, “The two will become one body.” But he who joins himself to the Lord becomes spiritually one with him.” 1 Cor. 6:16-17

“But he who joins himself to the Lord becomes spiritually one with him.” vs.17
“”For this reason a man will leave his father and mother and be united to his wife, and the two will become one flesh.”This is a profound mystery—but I am talking about Christ and the church.” Eph.5:31-32