Sex change operations are treating a psychological disorder (gender dysmorphia) with a surgical intervention. The problem with doing that is that a lot of people are going to change their minds, more and more with a longer time scale. Calling it a perversion under those circumstances, guaranteeing a fairly large proportion of people who undergo it will regret it later, seems quite reasonable.

If it were reversible and fewer regretted it later, i'd be more charitable. I incidentally feel mostly the same way about bariatric surgery. Treating a psychological disorder with a surgical intervention, though in that case it just rarely works.