Out of all the assholes I've dated, not one has been married or already had a girlfriend that I know of.
I've never understand why anyone would date a married guy knowing that they were married. The chances of them giving up their wife and children to be with a girl they sleep with are extremely slim, and you're setting yourself up for heartbreak. And also yes, even if it did work out with them and they married you, who's to say you won't end up in the same position their ex wife ended up in?