Negative Commandment #133
Sefer Ha’mitzvos Ha’Katzar (The concise book of mitzvos)
By the Chofetz Chaim
This book lists the Torah mitzvos that can be observed today
Second Section – the Negative Commandments
133. It is a negative commandment "do not have relations with a harlot who he has not performed the marriage ceremony with"
As it states in Devarim (Deuteronomy) 23:18, "there should not be a kadesha among the daughters of Israel." A kadesha is an unmarried woman who abandons herself to harlotry, or her father gives her over to have relations not for the purpose of marriage, or her father gives her over to harlotry. If a man has relations with a woman who is a kadesha for the purposes of harlotry and not for marriage, then both the man and the woman have transgressed this negative commandment.
This applies in all places and at all times.