The second consideration strengthens the first, while also giving new food for thought. Paul is thinking of Genesis 2:24, where it is said that a man will become one flesh
with his wife (see also Matthew 19:5; Mark 10:8; Ephesians 5:31). The sexual union is a deep union between two persons. It cannot be treated as something trivial.
16 Or do you not know that he who is joined to a prostitute becomes one body with her? For, as it is written, “The two will become one flesh.”