Exd. 20:14

This is going to sound strange but this came up in a conversation. The text reads as follows. Exd. 20:14 Thou shalt not commit adultery.
This other person was saying that the OT does not condemn prostitution by calling it sin and it was not a sin to be with one or use one. I objected strongly and sited Exd. 20:14.
However it was pointed out that only deals with adultery not fornication. The Hebrew word does not seem to cover affairs between unmarried people. I was a little taken back about that. Is there anyone here with Hebrew understanding that can shed more light on this. I still argue intimacy outside of marriage was a sin under the law, but is there a clear word on it? I can't find any.
Also David was admonished and punished for his adultery, but he was permitted multiple wives and concubines with no evidence of being called for it. So is there any word that deals with this during OT times?
One more thing. I am not talking about the NT and what it says. I am asking what did the scriptures tell the OT saints on this matter?