Most Helpful Guy
No its not cheating. BUT, if it bothers you then let him know that you don't want no ratchet ass girls grinding on him. lol but seriously. communication is key in relationships or your both going to end up on different grounds and eventually break up.
Most Helpful Girl
I'd say so, yes. If he's too weak to tell her off, he doesn't deserve you. Instead, he should be assertive with girls like that. She probably had no idea that he had a girlfriend, so it's not her fault. It's his fault for not doing anything about it. I was in a relationship with a guy who constantly had girls flirting with him, and in particular a girl who wanted to get in between us, but he never told her off, basically leading her on and making her think that what she was doing was okay, and I felt so hurt. I wish I'd broken it off earlier, but he was my closest friend and I didn't want to let him go at the time.
You should let him know how it made you feel and see his reaction. If he gets defensive, it's probably not worth it to stay with him.