In some countries, sex selection is already done, although it's typically done after a woman conceives. In the U.S., this would likely only fly if people did so before conception, but it's still a very dangerous and slippery slope. India and China are two countries that are in major trouble because of sex selection. These countries skew heavily male, and this will, in time, hurt the population. Men will find it extremely difficult to find wives, since millions of women are simply missing. This type of situation, where we mess with nature and its balance, can only spell disaster.
Historically, most societies in the past and even more modern societies have generally favored male infants than female infants. That is, there is a generally preference for boys than girls. If the appropriate policies are not in place when the technology becomes more accessible to the generally population, it could have devastating consequences in terms of tipping the balance between the number of men and women in a society.