Is it bad to hang out with my male friend w/out his gf?

Well, as long as his girlfriend knows that it's just you two hanging out then I don't think it's bad. But if it feels weird for you, there's no reason why you should keep doing it. Just get more people to come out with you I guess so it won't feel uncomfortable. If you feel he's flirting, maybe you should have a talk with him and tell him to stop because it doesn't feel right.