Perhaps husbands try harder with sons and are more involved in their child's lives if it's a boy.
Frankly, correlation does not prove causation.
Funny this should come up, I'm not planning to have any children but I did ask the Cute Boyfriend, if we ever had a kid, would you want a girl or a boy? His response, "Don't you think we'll be putting enough pressure on our kid without the pressure of 'I wish you were a boy' or "I wish you were a girl?" WOW! I was impressed. Especially since I would only want a girl.