Let me start by reiterating: I am very strongly against transgenderism. It's bullshit. However, there's a separate, civil-liberties case for allowing SRS anyway. This is a different discussion to whether transgender claims are valid. It's like the distinction between our liberty to reject a religion and the liberty of people to follow it themselves.

Do you think that cosmetic surgery to make people more sexually attractive ought to be illegal? Is that really reasonable? It makes them happier, that's proven.

Why should the fact a sexual fantasy figures into it, be a show-stopper? Isn't there a liberty to enjoy your own sexual fantasies?

More controversially, how about non-necessary abortion? (The Hippocratic Oath that contains "do no harm", actually does forbid abortion).

"Do no harm" does not determine medical ethics today. There are other criteria, like autonomy, consent, risk and quality of life.

It is unethical, it is immoral

Neither of these things imply it should be illegal though. Example: cheating on your spouse.

Now, let's say I concede all of your points above. Nonetheless, there are people who still want SRS, and they will do anything to get it, including going to dangerous backyard surgeons. Isn't this a bit like the problem with conservatives banning abortions? You aren't going to stop them, just make it more dangerous.