Commentaries:
Physically, you do not belong to yourself exclusively! If you are single, you belong to the one you are going to marry. And, even though you may not know that person, they have authority over your body. It is of such seriousness (see Deuteronomy 22), it can lead to the defrauding of that person. What is wrong with sex in this case is the timing. It is to wait until it is within the marriage bond.
In verse 4, another translation of "have authority over" would be "have rights over." Paul says we are responsible to give them what is due. The conclusion is that neither person—man or woman—has the right to use his body completely as he chooses because of the responsibility to the other person, married or unmarried. Even though a person is unmarried and may not have even met his future spouse, the latter has a vested interest in his body. Fornication is a defrauding of one's future mate because the fornicator is not using his body in the right way.
John W. Ritenbaugh
Passover and I Corinthians 10
The spirit of God's law reveals that all unchaste sexual conduct before marriage, commonly called fornication, is infidelity, unfaithfulness committed against the future mate. Our bodies are not our own. Could God have someone marked out as a mate for those who are single? The fornicator is denying him or her a pure gift. A vital, spiritual reality underlies this principle, for sexual purity is a type of our spiritual relationship with Jesus Christ, our promised Bridegroom.
John W. Ritenbaugh
The Seventh Commandment
Other Forerunner Commentary entries containing 1 Corinthians 7:4:
1 Corinthians 6:15-17
1 Corinthians 6:15-17
1 Corinthians 11:3