This is exactly the question. One question I don't see gets asked enough is, why, after so many decades of transsexual people being legally recognised in society without any pushback is trans inclusion suddenly such an issue?

One answer is that the number of people with "trans" (as opposed to transsexual identities, have skyrocketed in the past few years, and almost no effort is being made to differentiate between simple gender non-conformity and genuine gender dysphoria.

But the main answer is that lobby groups are going to great lengths to remove all of the previosly existing safeguards and assesments required to be legally recognised as a woman. Ten years ago, a male simply declaring himself a woman and walking into a woman's changing room or applying to be transferred to a women's prison was unheard of. Today, we're supposed to pretend there's nothing at all wrong with this.

The pushback we're seeing today is, I believe, a result of the push to remove the common sense "immigration policies" that were in place.

So when you ask, who gets to decide the immigration rules for womanhood, I think the problem is that trans lobby groups are demanding that they get to decide. And that anybody who says, for example, "don't you think there are safeguarding concerns related to letting males with no history of gender dysphoria into female prisons?", is a bigot who hates trans people.