1 Corinthians 6:15
Know ye not that your bodies are the members of Christ? shall I then take the members of Christ, and make [them] the members of an harlot? God forbid.
Note 10 at 1Co 6:15: All of these verses (1Co 6:14-20) are a continuation of the argument Paul gave in 1Co 6:12 for holiness (see note 8 at that verse). Since we are saved by grace, our actions are not essential for salvation, but God forbid that any of us should take that as a license to sin.
Our bodies are betrothed to the Lord and will someday be resurrected with Him (1Co 6:14). Therefore, we should view ourselves as being married unto the Lord and any action contrary to His Word as adultery. When we understand this union, we should not violate ourselves and our union with God through sexual immorality.
Note 11 at 1Co 6:15: Paul was saying that we are part of Christ's body. This is what the term "members" refers to. He used this same word to refer to physical parts of the human body (Ro 6:13, 19, 7:5, 23, 12:4; 1Co 12:12-26; Col 3:5; Jas 3:6, and 4:1) and believers as being part of Christ's body (Ro 12:5; this verse; 1Co 12:12, 27; Eph 4:25, and 5:30).