Nao's manga / #Shoujo(12)

I think this story seems pretty interesting... But I don't get why he should break up with his girlfriend. I mean the gender dysphoria is his problem not the sexuality, am I right? His gender identity has nothing do with his sexual orientation, so he can also like girls although he is girl in his heart. And he did say that he likes his girlfriend so I just don't understand why he's still gonna break up... Or does dating a girl and having sex cause gender dysphoria for him? And is it that he actually likes boys...?