There's a difference between being PC and pretending not to notice the cross dresser is really a man disguised as a woman or pretending not to notice a prosthetic vs. proclaiming some absolute hogwash like "pregnant man" and "man gives birth."
Plus your comparisons are invalid. When one aims to correct a physical deformity/injury/non-working part with something that works there's no reason to question the sanity of said person.
When one takes healthy human parts and opts to have them mutilated, chopped off, altered etc when there was never anything wrong with those parts to begin with that's when you can reasonably start questioning sanity.
This doesn't just apply to gender confused folks either. I equally question women who allow a knife to be taken to their breasts to insert man made parts to create an illusion of bigger breasts. People who cover themselves from head to toe in tats. People who create those really hideous huge holes in their earlobes, etc.
Obviously there are social and cultural norms and they change over time so I know much of the bias is relative to me. I don't mind regular earrings, a few tasteful tats, etc.
But there does come a point where the surgical manipulation starts to seem crazy, self destructive, and irrational. Breast implants and dick removal would certainly fall under those headings in my mind. The absolute destruction of healthy tissue in favor of creating a delusion seems irrational. Of course their sanity will be questioned.
When self improvement comes complete with hints and evidence of self hate there's clearly underlying problems and it becomes obvious no knife, no amount of cutting, is going to resolve them.