Is it normal to masturbate to girls when i have a boyfriend?

Okay, I'm not lesbian, but I've had sexual experiences with girls before. I'd say I'm bisexual. I'm currently in a great relationship going on two years, and the sex is still amazing. Yet I find my mind trailing to lesbian fantasies when I'm touching myself. I feel guilty afterwards. Like I said, my boyfriend and I have amazing sex and he turns me on, but I have a hard time getting off to him when I touch myself.