I don't think that happens..like at all.. I mean a guy that isn't your BF has no fricken right to tell you who to ,and not to, sleep with. And I don't know any dude that does that. Are you talking about a guy that your just starting a relationship with? cos in that case, its still a weird situation, cos he is either waiting to see which one of you ( yes he has an eye on a girl other than you too) will work out better. You wanna be the chick he falls back on?