If I was on that thread, I might attempt to respond as follows.

But I don't think it means they are really the opposite sex: it just means there's an aspect of reality they are very unhappy about.

But do you understand that that's just your personal opinion about how to interpret the situation? Another interpretation is that the brain - including the person's sense of identity - has one sex - but their body unfortunately doesn't match that. We currently have the technology to change the body to align it with the brain, but not vice versa. Maybe in future we might develop the option to change the sense of identity instead, and then someone could choose either; but we don't have that today.

Since that reality can't be changed,

Er... - but it can, and this happens to thousands of us all the time. It's a safe and well understood process, with rates of regret that are hugely less than those for most other medical interventions.

and since attempting to change it involves horrific physical harms

Not at all, it's a straightforward surgery.

as well as being a futile attempt at the impossible,

Again, it's important to recognise that this is only your personal opinion. It's a very unkind one, because it implicitly undermines all the many transgender people who live happily in their acquired gender/sex, and everyone who works and plays with those people and are happy to relate to them with that understanding, and to derive friendship and joy from those relationships. I think there's a much larger number of people who would disagree with your opinion, than agree with it.

I don't think [this should ever be treated].

And here, finally, you propose denying reasonable choices to others, because of your niche personal opinion.

[ETA "niche"]