1 Corinthians 6:16-18

Aren’t you aware of the fact that when anyone sleeps with a prostitute he becomes a part of her, and she becomes a part of him? For it has been declared: The two become a single body. But the one who joins himself to the Lord is mingled into one spirit with him. This is why you must keep running away from sexual immorality. For every other sin a person commits is external to the body, but immorality involves sinning against your own body.