Sex is marriage
What is marriage. Jesus defines adultery as a serious sin so then it is important to define what marriage is. A lot of people today define marriage as people married in a church with a marriage licence from the state.
I would say that the bible defines sex as marriage. There is no contract or state licence that defines it. Once you have had sex you are one flesh so therefore you are married.
Living in cohabitation with a woman is not fornication, it is marriage.