Not really a trap, kind of.
The first choice to be on her path or not, is to have sex with her. But of course, it doesn't happen, it was just a joke. A few lines after that, she goes to the shower, and you can clearly see she has a penis. So, just scroll back a couple lines and reject her instead.
The reveal is literally right after you make the choice to be on her path. So, like I said, just scroll back and reject her. MC doesn't do anything with her if you accept her any ways, so it's not like you are "tainted" or anything lol. Just showing interest is all. But a few frames later you can see she has a penis in the shower, so you can just go back and nope out of it lol.
So, yeah, it does kind of suck that it isn't revealed before you make a decision, but it's pretty quick to let you know afterwards so you can go back and reject instead.
For the player, it is frustrating not to know ahead of time. But story wise, it makes sense that you wouldn't know yet. There are little hints here and there that she is trans, but for some, it might not click right away.