Same boat as you. I'm fascinated and confused by this subgroup of people as well.

Couple other ideas I'll summarize, adding to yours:

• Yours: Young gay person tries to reconcile a) their internalized homophobia ("I can't be a gay person") and b) the message that "straights are evil," which on Tumblr is both pervasive and powerful. And then... somehow concludes that they're gay but NOT gay as a member of their actual sex? (I might add to this: I was taught growing up, myself, that "gay men are fun! Dazzling! Your ideal bestie! and lesbians are mean predators;" maybe some of these young women were also taught positive and harmful stereotypes about gay men, and negative and harmful stereotypes about lesbians? I'm grasping at straws here...)

My other guesses:

1. Dissociation from bodily experiences - Some research suggests that GNC people ("masculine" girls, "feminine" boys) are more likely to be bullied, sexually assaulted, etc. than people who are more "gender-conforming," and homosexual/bisexual people are more likely to be GNC. Both sexual assault, as well as shame about being same sex attracted, may lead to avoidance of thinking about one's own genitals, sexual arousal experiences, etc. People with this symptom might well miss entirely what their body is telling them, and then decide their "sexual orientation" purely on the basis of peer pressure, some article they read on Tumblr that made them feel accepted, etc. Maybe their body is screaming "no" but they wouldn't hear it.

2. Paraphilia - I think it's possible to be homosexual and have a paraphilia that is validated by the opposite sex. Ray Blanchard has talked about this before a little, and there is a data analysis blog covered a similar topic.