I understand the "it's not cheating if it's with a girl" sentiment, but:
1. It doesn't apply if she's doing it secretly/behind your back and hiding it
2. It Doesn't apply if you don't get to watch/pics and video/join in
Like for example, if your girl lets you pimp her out to randoms for cash, that's totally different than finding out your girl sucks c0ck for crack on the side when she's not with you