It is the syndrome most girls get (but some boys too) after they "make love" for their first time. And it doesn't matter if the dude/girl they did it with is the biggest loser/idiot/asshole whatever, they will give them every chance in the world no matter what other people say.
Girl: hey, I think I'm gonna give my x another chance.
Boy: Damn girl are you crazy?!? Listen I gotta know somethin. Did you give it up to him?
Boy: Aahh damn girl thats why you keep giving him chances, you got that First Fuck Syndrome!