John Calvin on 1 Corinthians 6:17

17. He that is joined to the Lord. He has added this to show that our connection with Christ is closer than that of a husband and wife, and that the former, accordingly, must be greatly preferred before the latter, so that it must be maintained with the utmost chastity and fidelity. For if he who is joined to a woman in marriage ought not to have illicit connection with an harlot, much more heinous were this crime in believers, who are not merely one flesh with Christ, but also one spirit Thus there is a comparison between greater and less.