My girlfriend and I have been together for over 3 years and I have always only loved her and only desired her physically or romantically. Recently she asked me if I thought other girls were beautiful besides her and I told her yes, that I had eyes and I told her that it was also different to see a pretty girl than a pretty man because I’m heterosexual. That being said, she accuses me of feeling attraction and sexual or romantic desire for these girls, just because I find them beautiful and visually “pleasing” to me. I’m sure and I’ve already tested that I can see a girl, find her beautiful and not have any desire to be with her on any level, but my girlfriend can’t understand that and says that since I’m a 21 year old man, because of the hormones I have to feel that way and now she wants to break up with me because she thinks I’m lying to her and because I’m attracted to others, when I’m sure this doesn’t happen.
How can I explain to her that she can trust me and that I don’t necessarily have to feel attracted to a girl just because I think she’s pretty?
TL;DR: my gf wants to breakup with me because she believes that if I find a girl pretty I’ve to feel attracted to her and me rejecting that theory makes her think I’m lying.