The only reason why parents might prefer boys is that teen pregnancy is more a burden on the female side. OTOH, teen males are more likely to drive crazy, etc.
On the “positive” side, I suppose parents may be (subconsciously?) comparing (statistical) “income potential”?

http://blogs.discovermagazine.com/80beats/2009/03/20/finch-mothers-can-subconsciously-control-the-gender-of-their-little-ones/