Fair question. And in response, I'll ask: How many leagues do you intend to have?
As many as needed. As many as interest can support.
For full clarity like I've said before we're mainly talking about professional or at the very least organized sports. If 10 random people want to get together at the park and play a pick up game of sportsball where's there's no "stakes" (for lack of a better term) involved I don't think here anyone really cares how they divy up the wiener roast and the clam bakes.
So for the purposes of this discussion when I say "sports" I'm talking organized sports that are sanctioned, tracked, metric'ed or otherwise have have stakes.
So with that clarification we have to factor in two things, both sorta the same thing. Interest in the sport and money.
If we change X factor about sports and nobody watches the sport, nobody wins in that scenario; the men, the women, the cis, the trans, the butchers, the bakers, or the candlestick makers. If people don't want to watch biological men versus biological women or don't want to watch a league that doesn't let people play as the gender they identify as or doesn't want this or doesn't want that, however this eventually plays regardless of your, mine, or anyone's personal opinions about it. If nobody wants to watch Football because Football isn't fun to watch anymore, then who gets to play football in what combination become a meaningless secondary question real quick.
And even sports that aren't "professional" in the literal "this is a business and this is how they make money" sense still aren't going to work if nobody cares about them or wants to watch them or is engaged in them.
So a BIG question here has to be "What do sports fans want to watch?"
And I'm sorry but right now, with the information we have people don't want to watch women's sports. The largest female sports league in the world is the United States National Women's Soccer League and it's 65th in the world, behind dozens of men's leagues that me, you, and everyone in this thread would be lying if we said we had even heard, much less were engaged fans of. I'm pretty sure there's no hardcore fans of the Carribean Premier Cricket League in this discussion and it's more popular than the most popular women's league. By several ranks.
Given the simple numbers involved it's mathematically safe to say that WOMEN want to watch male sports more than female sports.
Or even more meaningfully right now in North America there is... like just so much football. Like a lot of football. The NFL is the largest sports league not just in the US but in the world by a fair margin. College Football is, let us cut any pretentions of crap and just call it what it is, a major professional sports league with a diploma mill system attached to it. High School Football is a defacto pro-league in large chunks of the country. Arena Football, Indoor Football, hell they are taking a SECOND stab at the XFL.
That's a lot of football. We love football. We can't get enough football.
How many professional women's football leagues are there? One. Lingerie Football. I'm not making that up. There's 74 High School Football Leagues for boys. There's one for girls.
You can't force engagement. This isn't like hiring practices at a business or equal opportunities in education or even disparage in elected officials. You can't force the kind of voluntarily engagement an entertainment like sports needs. You can (to some degree, massively oversimplifying this) force equality when needed in things that matter but this is sports. It's purely voluntary entertainment. If people don't want to watch women throw a ball or kick a ball or shoot a puck around the field as much as men you can't make them.
Which brings us to your next point.
When you sort leagues in this way for athletics, whether it's intentional or not you end up excluding females from participation. Remember that the super-elite pro female soccer teams routinely get beat by middle school male teams.
Yes. This is true. And I can't help it, I can't fix it, and I can't make it not true. I'm sorry. I've talked about at length in this long, long, long, interminably long discussion about the "Can't win, can't lose, can't break even, can't quit the game" and "Impossible trifecta" situations this discussion has put us in time and time again and I'm sorry but "Oh and by the way the solution ALSO has to let biological women maintain some illusion that they are physically equal to men IN THE WAY SPORTS BASED PERFORMANCE IS MEASURED" is... no. Sorry.
If, under the "just sort by performance" concept I presented, the "market" can support, let's say a Major League, a AAA Minor, and AA Minor Football League and all 3 of those leagues are 100% biological male outside of maybe a few scattered exceptions here and there then... sorry. It's not even a matter of me liking it or thinking it's "right" in any useful or meaningful sense of the term it's just not a problem I or anyone can solve.
"I get to play sports in an organized league with a TV Deal and a sponsorship with Nike and my face on Wheaties box" isn't like access to abortion or equal pay.