There are many sports leagues where what is considered to be the mens league, is actually open to women, it’s just that no women have ever competed in those leagues.

(If this is because of biological differences, or because of patriarchal systems (or both) is up for discussion, though I think patriarchal systems do play a massive role)

In baseball for example there have been some women playing professionally for minor league teams, and some time ago for the first time in D1 college baseball.