Is it normal to fantasize about being the other gender?

I'm a 16 year old, and like every other teenager I fantasize a bunch about sex and relationships and stuff. Thing is I like to imagine myself as the girl a lot even though I'm a guy. It's pretty random actually because sometimes I imagine myself as a girl and sometimes as a guy. The gender of the person I'm with also changes randomly but that's expected because I'm most likely bisexual. I'm not transgender because I like being a guy but sometimes I just see myself as a girl. Also I don't just do it occasionally as a passing thought. I'd say it's about a third of the time I think about future relationships/sex I'm the girl. So what? Is it normal for gay/bisexual or even straight people to fantasize about being the opposite gender or is it just me?