The lack of permission issue is muddy because you specify that it's what they "want", which implies some kind of magical hypothetical permission happened somehow. I won't address it.

A concern I haven't seen raised is the people who live in cultures where transgender issues or even just being a particular gender could be life-threatening. I would be worried about the people "outed" unexpectedly. Some trans people might decide to forego transitioning due to other concerns in their lives weighing higher, and I would want them to be able to weigh in on that.