Sex with a single woman would not be adultery?
Nowadays that people aren't makpid on tumah and tahara, basically every girl from around bas-mitzvah age is ervah (and there's a gezaira, Rabbinic Decree, that they don't go to the mikvah until married) and included in the prohibition of immoral relations, arayos, adultury, or whatever you want to call it. So unless you want a very young single girl, every single girl is included and forbidden.