Charles Ellicott Commentary 1 Corinthians 6:16

"Or know ye not that he that is joined to a harlot is one body? for, The twain, saith he, shall become one flesh." — 1 Corinthians 6:16 (ASV)

What?—As if someone might question and resent the strength of the previous words, and wish them “watered down.” “Do you not know that my strong assertion is true? It is not merely my statement; it is to be found in the Old Testament, ‘Two shall be one flesh.’”

This was originally (Genesis 2:24) applied to marriage, as showing the intimacy of that sacred union, but here St. Paul applies it to one aspect of a union which, in one respect, was identical with marriage.

Of course, the other parts of the Apostle’s argument do not apply to marriage, the union being a sacred one. Two becoming one flesh in marriage is no degradation of a member of Christ—indeed, it is a sacred illustration of the complete unity of Christ and His body, the Church. (Compare 1 Corinthians 11:29, and Notes there.)