Adultery creates a second one-body/one-flesh bond in opposition to the marriage. This will inflict severe damage upon the marriage relationship. The apostle says such sexual sins hurt so much because they are "sins against [our] own body" (verse 18).
Paul comes to his primary point in verses 19-20: We are not our own! God bought us at an incredibly high cost, the blood of our Master, and thus He commands us to "glorify God in your body and in your spirit," both of which are His! God owns us completely!
The import of this is staggering! When we commit sex sins—even in our minds—we have first become unfaithful to God! When we break the seventh commandment, we show infidelity to God! Yes, it shows infidelity to the wronged spouse, but it all begins with unfaithfulness to God.
The road to adultery starts when we become willing to break the vows we made to God at our baptism. We promised then that we would honor and obey Him exclusively and faithfully, accepting Him as our Savior, Master, and soon-coming King and Husband. When we are willing to walk away from the commands He gives us about sex and marriage, we begin to walk into the arms of adultery. Physical adultery starts with spiritual adultery!
If an adulterer desires to repent, he must first acknowledge that he has sinned against God. King David, in his moving prayer of repentance after the murder of Uriah and adultery with Bathsheba, cries out, "Against You, You only, have I sinned, and done this evil in Your sight" (Psalm 51:4). Did he not also sin against Uriah, Bathsheba, the nation, his wives, and his children? Of course! But ultimately, his sin was against God! When we are faithful to God and our covenant with Him, we will not commit sex sins.