

There is no resonable way to do that.
The only way I could see it being done while perserving some kind of anonymity would be to have a defederated instance, where members are invite-only, where members can invite their friends to join, there by verifying that there are no men joining.
Unfortunately, there is no good way of verifying the gender of a person that you don’t meet outside of a forum.
Sounds fantastic, I hope that you can find a community like that again.
Vetting the post history is probably the only realistic way of doing it, it is time consuming, I am sad that it is needed, but if that is what it it takes it is at least doable.
I wish you and all women online good luck in finding a community like that, everyone needs a good community to enjoy!