Sexuality ?

I've always known I'm not fully straight but I've always struggled trying to figure out my true sexual orientation. I've been with my boyfriend for over 2 years now and i can honestly say i do very much love him. There's one problem. Most of the time when we have sex i close my eyes and picture girls..and it makes it easier for me to get an orgasm. Don't get me wrong our sex life is amazing but for some reason i always have girls on the back of my mind. Is this wrong ? Is it mentally cheating? Maybe I'm over thinking things but I'd like to get some feedback. Has anyone every gone through this before?