Biological sex is a physical category. People are assigned to physical categories based on physical evidence. There is no possible mental measurement that could assign someone to a physical category.
And trying to assign someone to a physical category based on mental health claims (“It makes him happier!”) is an emotional plea, not a rational argument. It ignores the impact this action would have on others. They matter too.
I oppose wrong beliefs and ideologies, not people. Had I walked in another's shoes, I might believe and do as they believe and do. I don't oppose people. I seek to persuade them. Wrong beliefs and ideologies are forever wrong, but a wrong person can choose to become right.