First of all, most women can't orgasm from 'normal sex' so that is probably why she prefers mutual masturbation, she probably can't orgasm unless her clit is stimulated. If it bothers you, you might be able to find new creative ways to have sex while stimulating her clit, and that might be enough to satisfy her. Since you can watch her masturbate, you should know exactly how she likes it, and it should be easy to replicate yourself.
Do you guys watch porn together? I would not assume that her interest in the female body has anything to do with her being a lesbian, because she's with you. Has she been with females in the past? She is obviously into you because she's having sex with you. She may be attracted to females, or she may just enjoy looking at porn. And frankly, there isn't much good porn with men in it- it's almost all centered around the women because men mostly buy porn. But if she does find women sexy, so what?
I think women can be very sexy. I am 100% straight. It sounds like your relationship is great and there is nothing to be worried about.
Anyway, why don't you just ask her?