Quoted from CrazyLevi:You are not allowed to play in a women's league because you have a penis. This offends you. Are you offended that girls can't join the boy scouts? Are you offended that your son can't play on a women's softball team?
Eh, we got rid of that in Canada (and most of the world) years ago. (When I was still in Scouts.) But the girl scouts are still around, if less popular, for various reasons.
And in terms of 'coed' sports, Ultimate has rules requiring at least 3/7th of players on the field to be of either gender in mixed leagues. This leads to huge participation drives to try to find enough girls/boys when you're putting together teams, and I think a better environment in general.