A.T. Robertson 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)

One body (εν σωμα). With the harlot. That union is for the harlot the same as with the wife. The words quoted from Ge 2:24 describing the sexual union of husband and wife, are also quoted and explained by Jesus in Mt 19:5f. which see for discussion of the translation Hebraism with use of εις.

Saith he (φησιν). Supply either ο θεος (God) or η γραφη (the Scripture).