Well, being a mom with 3 girls, we definately have more female offspring! I think genes definately come into play,also....some men might have more xx sperm, or visa versa, they have more xy sperm. But a thing I have heard of, however, is how close to ovulation you have intercourse predicts the sex, or the more chance of having a XY or XX. Having intercourse 3-4 days away from ovulation can increase having a female, because female sperms live longer, thus increase their chances of reaching the egg, and the male sperms die faster. Having intercourse closer to ovulation, like a day before or on ovulation day, increases your chance for a male, because male sperms are faster to reach the egg than female sperm (but they die off quicker than female sperms, so it needs to be around ovulation day). Many people swear on this theory, there is a book called "shettle's method" that explains it more in detail.
Good luck with your class!