Women’s sports were explicitly created for women. Most men’s leagues allow women, but at the pro level there really aren’t women that can compete at the same level.
I think the focus on winning or trans athletes dominating is overblown and ultimately irrelevant. A Trans women winning an Olympic gold probably never happens, not because of biology, but because it’s mathematically unlikely.
For competitive leagues I think the fairness question is who gets the last spot rather than who wins. Is dropping a cis women for a trans women fair? Is allowing a cis women who took PEDs for a prolonged period, but has been clean for a few years fair? I think it’s reasonable that the answer should be the same to both.
For non-competitive leagues I don’t think it really matters at all.