It’s a Victorianism syndrome,

oxfordreference.com/…/acref-9780195176322-e-1665

Poor Law Amendment Act of 1834—were introduced for the moral reform of paupers

Where morality has nothing to do with actual morality like in whether it is doing harm. But everything to do with government making decisions over your private life and your body.

This mindset is disgusting, immoral and childish. And it so permeate everything about most English speaking countries culture for some reason.
It’s a kind of a thought police, that has no base in actual morality. But in bodily inhibitions and insecurities, supported by superstitions about an old man in the sky who disapprove.

So anything having to do with naked skin and sex is a nono. Even if it’s not actually sex but only loosely related to it, like being gay or trans.

If they were really serious about this sex thing, stating to be CIS should be exactly as problematic, and pushing the thought of marriage on children would be extremely immoral, because that too indicates sex. Which has almost become the sole purpose of marriage under the moral standards of Christianity, as it’s the only allowed form of sex.

So in short, the answer to your question could be boiled down to “brainworm”.
There is no logic or moral explanation, that doesn’t boil down to inhibitions, control and religion.