I would have to dig through the archives to find the thread, and we all know I'm not going to and it would be pointless too because it was the MOST EPIX BF thread of all time it would take hours to dig through, but Paul makes it pretty clear that sex with a harlot does form a "one flesh" relationship and he uses the exact same phrase that Jesus uses when he addresses marriage and quotes the Old Testament.
Here it is:
I Cor. 6:15,16: In the context of explaining the gravity of what it means to be a member of the Body of Christ, Paul invokes prostitution to refer to that which one should avoid, continuing [CLNT], "Or are you not aware that he who joins a prostitute is one body? For, He [Yahweh] is averring, the two will be one flesh."