Trans men are trans men. Men are men. But we call both 'men' for convenience and out of respect and acceptance of a social reality that is experienced. But they are not entirely the same thing, even if we use the same word.

All men are men, but not all men are the same. Some men can get pregnant, but most men can't and most of those men were born as men.
Not all women can get pregnant either.

The ability to pregnant as a defining property, is just too limited and just doesn't work anymore. Things change.

Also, wouldn't it be horrible to have to ask a woman if they can pregnant, just to make sure they're a woman? Same goes for the other way around.