How about this: Once a male user posts, he temporarily becomes a female user until three more male users post again.
So it's this:
Marstar2000 is temporarily female (he starts counting) until three more male users post.
I post. I temporarily become female.
Two more male users post. Marstar2000 is back to being male and these male users temporarily become female users.
This way, it's more fair to the girls because there are a lot less girls than boys. And remind an user when he is back to being male.
Also, use male pronouns even if a male user becomes female.