That might depend on the event, but the first thing that came up disproves your blanket claim. It's from World Boxing:

  As part of this policy, World Boxing will operate two categories as determined by sex: a men’s category and a women’s category. To be eligible for the men’s category, a competitor must be male at birth. To be eligible for the women’s category, a competitor must be female at birth. [0]

This is for the safety of women. You can comfortably ignore the mad bit where they add "at birth" as though you can change your chromosomes.

> edit: there's speculation about women eventually crowding out men in ultra-marathons. Then there would be a good reason for a men's category.

I don't think so. No one cares about male competitors who aren't the best of the best.

[0] https://worldboxing.org/wp-content/uploads/2025/08/World-Box...