Strong moral men get boys.
Not so strong yet moral men get both boys and girls.
Weak Metro's get girls
I don't necessarily agree about the first two you mentioned (morality making the difference), but I definitely agree with this one. I have noticed it for many years, going back to my childhood. I didn't think of weak metros, though; I thought of it in terms of the mother: women who wear the pants in the family are more likely to end up having girls (and often, only girls). I've seen this with lots of families over the years. It's definitely not 100%, but it's noticeable.
This is absolute absurdity