They have a mental problem, not a physical problem.
Or, they have a fetish for modifying their body. But no amount of modification can change your gender, which is defined at birth.
It is sexist to say that any man or woman has to think, feel or act a certain way. When a man “feels” like a woman, it means they’ve been programmed with a sexist notion of what a man should or can be. No thought or feeling is inappropriate for either gender to feel, therefore no feeling you have should give you any reason to think you need to “change” your gender with surgeries.
Both.
When a man feels like a woman, it means he has been programmed with a sexist notion of what a man should or can be.
That insight also works if you substitute "woman" for "man."