I'll just comment on the thing i bold out since it's where I have some experience (being mentally ill myself).
There is a lot of things wrong with having a mental illness. It's a illness for a reason.
Although you probably mean to say "what is wrong with thinking someone has a mental illness?", well, it in itself can be meant in a non-offensive way, but the receiver can and likely will see it as offensive since they don't necessarily agree that it is a mental illness.
A mental illness also often has diagnosis within the field of mental health or will eventually get there; being LGBTQ has been there but been moved out of that and has still had to deal with being taken as a mental issue by people with views like Shapiros. Shock therapy and I'm sure worse has been historically used as a way to treat such "mental illnesses".
If someone is misdiagnosed with a mental illness, it's a horrible from lots of points of view, like self-reflection, being branded by society or mistreatment that can lead to more/actual issues.
Mind, I'm not trying to say that people of mental illness should have to diagnose themselves as that would be a weird way to solve it on, but surely stuff that we as a society can agree to be a view of the past and have as a society moved on from we can make the effort to better even if we don't necessarily understand it ourselves.
I'm one of the people who genuinely struggled to understand that people used gender and sex as different-meaning words.
I've had chats with folks like
@DiseaseOfTheAge and tried to clear up my misunderstandings on the topic and try my best to not use them wrong in the future.
I very much doubt that no one has ever told Shapiro that the view is that sex is what you are born with and gender is how your identity is viewed by yourself and society(or something to that effect, sorry DoA if i missed the last one), which would make his refusal to accept the splitting of those terms only really hurtful.
Him calling it a sin from the perspective as a religious person i can accept, not that I'd accept it as a sin but I can accept him thinking of it that way from his belief as long as he doesn't act upon it and negatively alter someone elses life. After all it's whoever was by the pearly gates or god himself who was supposed to do the judging from what I can remember from my lessons in youth about most of the large religions.