Are you guys serious? He is saying that he feels torn. He doesn't feel happy about what his girlfriend does, yet you guys tell him to get out of his comfort zone.
Buddy, my advice is this: I know it's been a while since you began a relationship with her, but are you sure you want this? Do you see yourself as being happy later on? If your answers are negative, then consider breaking this tie. If your sexuality and hers aren't compatible, and the compromise is too much for you, don't force yourself to stay in that relationship. It'll cause more stress and frustration.
I find it endearing how everyone tells guys to accept their girlfriends' sexual desires and get out of the comfort zone. But if the OP was a female, I bet everyone would be advising her what I am advising this good sir right now.