I have no idea whether this is true or not but my sister (who is about to have a baby girl) claims it's all about timing. Gender is determined by the sperm, and apparently girls are slower but live longer while boys are faster but don't survive as long. So if you have sex on the day you are supposedly most likely to conceive (in terms of ovulation) then the boys are likely to get there first. However if you do it a couple of days beforehand, the girls will survive and still be around to fertilise the egg when you ovulate, after the boys have gone.
As I say, no idea if it's true or not, others may have an opinion!!