Just because we don't see eye to eye on just about everything doesn't mean I enjoy seeing you be treated like dirt. She's obviously ashamed of either you or her friends. It's possible she is cheating but maybe not. I know it's out of the question to actually sit down with her and ask her why she never includes you, that would be crazy so if the sex is good, just ride it out until she either falls for the other dude and sees it as a real relationship and can break it off with you, or you find a woman who thinks enough of you to include you once in awhile in her circle of friends.