I see no problem with it. It's already happening really, although it isn't a perfect science. They have studied sperm and have found that certain sperm make male babies and certain make female. And the ones that swim faster but die sooner make a certain sex ( I think it's male but I could be wrong), while the ones that swim slower but live longer make the other. So, knowing this, a couple can help push the odds in their favor of having the sex they want by what time they make love compared to when she ovulates.
For example, if they waited to have sex until the day she ovulated, she would be more likely to have a male child due to the faster swimmers reaching the egg first. But if they had sex a day or two before she ovulated, the fast swimmers would die off and the slow--yet long living--sperm would make it to the egg and make a female.
Don't quote me on that because like I said, I might be mixing up which sex is which. My point is, there is always a way to bend nature in our favor.