
Maybe she feels bothered by it because she's had feelings for him for several years and now she feels fooled because he probably rejected her previously, stating that it was due to his sexual orientation. (⊙…⊙ ) I mean... If it were me, I wouldn't blame the guy I was in love with but I'd probably feel pretty heartbroken, knowing that he could supposedly date another woman (and not me). ╮( ̄▽ ̄)╭

Because he is lying to himself and whoever the woman is. Not sure why anyone would think a person who is gay, and identifies as such (as opposed to a 13 year old who is still unsure/struggling) going out with a woman is a positive thing. I guess decades ago, some people would feel societal pressures to get married to the opposite sex, but that is in the past, and a couple who are the same sex can get married and have the same legal rights and protection. He's essentially going out with her just for show, and to use her to get access to her brother.... no, not really on in this day and age.
Then again, the brother dressing up as a woman in order to see if he really was gay was fucking weird too,

OOPS! I think I'll be more careful about assuming the gender of characters, in the future... I'm not that mentally invested in this particular story, to begin with ... Well, the blonde person just ... kind of looks like a woman to me but I guess I was wrong; I understand why you're confused about his(?) reaction now. But of course, if you're convinced that you know something that is as integral to a person's character as their sexuality (and they've even stated it explicitly on several occasions), I think you'd feel betrayed and hurt if they act in a manner that would contradict with the self-image they'd depicted. And if he(?) could somehow sense a hint of dishonesty from him that would only give him all the more reason to react as strongly as he did emotionally.

Do you? According to wiki "Genophobia or coitophobia is the physical or psychological fear of sexual relations or sexual intercourse.". Not sure why you even mentioned this disorder as it has zero relation to the characters or plot. None of the characters appear to have any anxiety over sex, so it seems like your comment was just you confessing to having this phobia, which seems a bit pointless, but I guess if you found it cathartic to divulge personal info about yourself, erm good for you?
why does it matter so much if he dates a woman? aside from that the people around him knows he’s gay but still.