i fantasize about girls during sex with my boyfriend. am i lesbian?

i'm 22 and i've only ever dated and had romantic attraction towards men. the prospect of flirting with, dating, etc. a woman makes me uncomfortable and i dream of the day i can marry a man and have a family. however, sexually, i find i can normally only get off if i fantasize about women during it. this has been a thing since i was 17 or so. while my boyfriend gives me head, i'm fantasizing about a girl doing it. not any girl that i know in my life, but rather an anonymous girl/the idea of one. it feels taboo to me and i think that's what's turning me on. i'll pretend a girl is eating me out instead and then i'll cum, but if i don't pretend that i won't. i don't know if ive screwed myself up by watching lesbian porn most of my late teenage years or something? i'm very attracted to my boyfriend and i love being in a relationship with him (and just men in general) so i don't know what my issue is. can anyone tell me if this is normal?