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.
You do however, justify somewhat the "spread" of sexes in families, at least on a cursory level.
Then again, we both might be full of crap.
/8^)