Oh it's completely normal to have a preference one way or the other, and it doens't mean that if you got the opposite that you wouldn't love them any less at all, but sometimes it can be quite severe and you may need some counselling for it.

I think sometimes you prefer one over the other for various reasons - some want a first born son to continue the family name, others want a mother/daughter mother/son relationship etc. I really wanted our first to be a boy, not really sure why - partly because of the firstborn son thing but it wasn't definitively why I wanted a boy.