Just because one culture has a particular view doesn't mean that it applies universally. Gender views are far more fluid in Glorantha than in our world, but that also means that there are more gender restrictions. This is part of the complexity of Gloranthan culture, multiple gender types are accepted but also strong feelings about gender appropriate roles. Yelmalio is very masculine, Ernalda is female dominate, Trolls are fiercely matriarchal but there are always exceptions!
How would gender be determined? Beyond what the individual feels, when it comes to initiation rites its the God's that decide. If a supplicant is accepted then they meet whatever criteria is necessary. And if a player wants their character to join a gender restricted cult, it would be up to the GM to rationalise exceptions.
Its myth, men give birth and women father children!