Depends on many factors. If the girl had a crush on the guy prior, and the sex was not horrible, the crush will most likely intensify. Same is mostly true for the guy, but if the guy had no crush, and was just after sex, then the girl is expendable. Guys and gals view sex differently. For most gals it is like some super important thing. For most guys it is just fun...like a game or slow dance or whatever.