Because we've established that mind erections are a thing now. (It's like a regular penis erection, but for the brain, and available to all genders. Frequently caused by fandoming or finally figuring out the thing you've been trying to figure out for like, ever.)
Imagine a world where a certain very powerful group of leaders (the kind that make the rules and stuff, the sort that dictate society), are mostly males who claim to be straight. Now, to best defend or prove that claim, they show massive intolerance to everyone else. Not just to the homosexual community, but to straight women as well. (Which doesn't make sense, until you consider that maybe it's only a claim, and then it's obvious why they'd want to be unappealing to the very people they claim to want to attract.) In fact, some of them dislike women so much that they create a world where all the women fear being sexually assaulted. To the point that it's assumed it'll happen in some form to you, that it's just a given. Now suppose they outlaw homosexuality-- no marriages, unions, or any recognition of those relationships. So there are now, in this scenario world, a bunch of very horney dudes who are turned on by other dudes. Okay, so, one day an issue comes up about restrooms. Some people are identified as having a different gender than the one they were born with. Or maybe they have more than one gender. The details don't matter so much as the response. The straight men in charge declare only men in the mens restrooms, and only women in the womens. They even go so far as to claim that the gay males would sexually assault the women if allowed to poop in the same area.
Is it possible, in this scenario, that the powerful straight men might be the ones sexually assaulted in the mens bathrooms? Leading to 3 possible conclusions.
1- They want to fear that, like the women do.
2- They aren't actually straight, but fear admitting it, so they've created a scenario where in they can get what they want while simultaneously claiming not to want it.
3- They just aren't smart enough to think of either of these options. Being male, straight, or in charge does not mean intelligent or wise. It should, but this isn't that kind of world.
I'm just tossing it out there. Obviously this is the basis for a fictional world. I mean, come on, it's a real stretch of the imagination!