He has a girlfriend
No, no, no, no, no. A lot of women out there think that dating a dude with a girlfriend isn’t a big deal, stating, “It’s not like they’re married.” But all of the same rules and issues still apply. He’ll forever see you as a sidepiece. You give yourself no shot at any sort of real relationship with this man, since things started with such a sullied beginning. He doesn’t respect the sanctity of a relationship. Just get out of there.