The fact that when on a first date, a girl is less likely to sleep with someone she really likes and wants to see again and is more likely to sleep with someone who she never wants to date again. This can due to the belief that all her partner is looking for is sex, and once that is achieved he will move on. This can also be due to a woman having sexual drives but no one she is romantically interested in around, so she goes for an easy, convenient target.
"I held out on having sex with the sexy fireman after he took me on the best date ever because I really liked him and wanted to make sure we'd go out again - but I did sleep with my douche bag neighbor after a couple drinks because I realized I didn't care if we had another date. That's the first date fallacy for you."