I had a relative tell me that the problem with trans and homosexuality is that it's a mental illness. Instead of fighting those ideas, my response was "so what?" I decided to follow that line of argument to its conclusion.

Even if it were a mental illness, it would be one that causes no harm. I asked that relative what they would do if they found out they had a mental illness, and that the only thing that treatment would accomplish would be to make them no longer love their spouse. Of course they'd refuse treatment and go on loving who they love.

That should be the end of the conversation. If someone is doing something that harms nobody, why should they be forced to change who they are to please someone unaffected by the outcome?