Is it normal for them to hang out with girls in a club without their girlfriends? Is it like cheating? I really don't understand.
It depends. If they're part of a group of friends it can be okay. Sometimes a group of his female friends wants to go dancing, and they feel more comfortable if there's a guy around because he can help keep the creeps away. So it could be that situation.
It's also possible that he has a male friend who's single, and acts as the "wingman." This means that he'll go start conversations with girls or groups of people, and then introduce them to his single friend. Since he already has a girlfriend, he feels less nervous about approaching new people, since he has nothing to lose.
And sometimes people just like dancing and being social. Clubs aren't always about trying to find sex or a girlfriend.
But if he's there by himself, or if he's really flirting a lot with women or trying to dance with them suggestively, then that's not okay. It isn't the fact that he's at a club--it's what he's doing there!
I dn't understand why you say that,
can you examine me please