One of the most commonly-followed theories on gender-prediction is The Shettles Method (Info found here).
The idea is that male sperm are faster and smaller than those carrying the X chromosome, which are slower yet heartier. BDing within a day or two of O will give you a good chance of a male child while BDing a few days out will give you a better chance of a female.
One thought to your genetic question is that perhaos your and/or your DH's famiies Y sperm are extremely fragile, giving the X sperm a much better chance of hitting the egg, regardless of when sex occurs. If you *really* want to try for a boy, and aren't in a hurry to get pg, try saving sex for the day before & day of O (you'll have to be an experienced charter to figure that out, and there are plenty of women in the TTC with Charting threads to help you out.