Modern human mothers can now choose the sex of their children, without surgery or procedures. For every pregnancy, the sex of the fetus must be chosen. In a culture that has a strong preference for male children, such as China or India, how will that culture adapt to the ability to produce only male children?
So let's say this new ability is discovered, and immediately all the expectant mothers turn their fetuses male. Fast forward twenty years, and you now have an entire generation of men, ready to enter the workforce and change the world, yet unable to find a spouse (except the gay men, they've lucked out in this scenario). It's very likely these men have lived their lives with virtually no women their age. The vast majority of them are probably virgins, and many of them will not be happy about this. Building a family is one of the highest priorities of a lot of people, especially in male-centric cultures; these men are not going to like the idea of their names dying out.
So what will happen? I imagine a lot of wars. The armies of these countries can simply say "Join the army, meet some women", and enlistment will skyrocket. These armies will attempt to expand the borders of their nations, and take the women of the bordering nations hostage. It may seem silly to think that men will kill to get laid, but it is an instinctual urge; it's not as crazy to think that someone would kill to feed their family, but these men need to kill to have a family.
As for women, things are going to get very bad for them. You may think that they'll have more power since they have a valuable commodity, but you forget the thousands of years of history which prove you wrong. Women have always been a valuable commodity, and as a result men have treated them like slaves, taking away their basic human rights and selling them like property. If there are less women, the ones who are left are going to be treated even less like humans and even more like baby-making machines.
Put these two things together and you have armies of millions of sex-crazed misogynists. I imagine many other nations would band together to stop these armies, take over these countries, and free the enslaved women. Or maybe they'll just impose embargoes, it really depends on how bad this gets, and how quickly. The main problem is that it will get bad. Cultures that value male babies over female babies aren't going to value anyone who decides having a girl is a good idea when you have the choice. The only way they're going to adapt is when someone forces them to; whether that's by the UN after the country's government is toppled or by the all-male government so they can rape a new generation of girls, I can't say.