I'm terrified that I'm bi

Ok so I'm 18 and all my life I've been with guys. I've been with my current boyfriend for 3 years and I really do enjoy sex and being intimate with him, as well as just being in a relationship with him. But for some reason lesbian porn turns me on more than straight porn. I would never want to be in a relationship with a girl but I kind of think girls are attractive, just not enough to ever want to be with one. Does that mean I'm bi??? I would honestly never want to be for personal reasons, I only want to date and marry a man. I would be so upset with myself if I was actually attracted to girls.... Please don't call me a homophobe etc because I don't care at all if other people are gay or bi but for myself is a different matter.