For an idea to be truly dangerous, it needs to have a strong and near universal appeal. The idea of being able to choose the sex of one's own baby is just such an idea.
Anyone who has a deep-seated and profound preference for a son or daughter knows that this preference may not be rational and that it may represent a prejudice better left unacknowledged about them. It is easy to dismiss the ability to decide the sex of one's baby as inconsequential. It is already medically feasible for a woman or couple to choose the sex of a baby that has not yet been conceived. There are a variety of safe methods available, such as Preimplanted Genetic Diagnosis (PGD), so-named because it was originally designed for couples with fertility problems, not for the purpose of selecting the sex of one's next child. With PGD, embryos are created in a Petri dish, tested for gender, and then implanted into the womb, so that the baby-to-be is already identified as female or male before implantation in the womb. The pro argument is simple: If the parents-to-be are adults, why not? People have always wanted to be able to choose the sex of their children. There are ancient records of medicine men and wizened women with various herbs and assorted advice about what to do to (usually) have a son. So, what should it matter if modern medicine can finally deliver what old wives' tales have promised for countless generations? Couples won't have to have a "wasted" child, such as a second child the same sex as the first one, when they really wanted "one of each." If a society has too many boys for a while, who cares? The shortage of females will make females more valuable and the market economy will even out in time. In the mean time, families will "balance out," each one the ideal composition as desired by the adults in the family.
Every year for the last two decades I have asked students in my college classes to write down the number of children they would like to have and the order in which they ideally want to have girls and boys. I have taught in several different countries (e.g., Turkey, Russia, and Mexico) and types of universities, but despite large differences, the modal response is 2 children, first a boy, then a girl. If students reply that they want one child, it is most often a boy; if it is 3 children, they are most likely to want a boy, then a girl, then a boy. The students in my classes are not a random sample of the population: they are well educated and more likely to hold egalitarian attitudes than the general population. Yet, if they acted on their stated intentions, even they would have an excess of first-borns who are male, and an excess of males overall. In a short time, those personality characteristics associated with being either an only-child or first-born and those associated with being male would be so confounded, it would be difficult to separate them.
The excess of males that would result from allowing every mother or couple to choose the sex of their next baby would not correct itself at the societal level because at the individual level, the preference for sons is stronger than the market forces of supply and demand. The evidence for this conclusion comes from many sources, including regions of the world where the ratio of young women to men is so low that it could only be caused by selective abortion and female infanticide (UNICEF and other sources). In some regions of rural China there are so few women that wives are imported from the Philippines and men move to far cities to find women to marry. In response, the Chinese government is now offering a variety of education and cash incentives to families with multiple daughters. There are still few daughters being born in these rural areas where prejudice against girls is stronger than government incentives and mandates. In India, the number of abortions of female fetuses has increased since sex-selective abortion was made illegal in 1994. The desire for sons is even stronger than the threat of legal action.
In the United States, the data that show preferences for sons are more subtle than the disparate ratios of females and males found in other parts of the world, but the preference for sons is still strong. Because of space limitations, I list only a few of the many indicators that parents in the United States prefer sons: families with 2 daughters are more likely to have a third child than families with 2 sons, unmarried pregnant women who undergo ultrasound to determine the sex of the yet unborn child are less likely to be married at the time of the child's birth when the child is a girl than when it is a boy, and divorced women with a son are more likely to remarry than divorced women with a daughter.
Perhaps the only ideas more dangerous that of choosing the sex of one's child would be trying to stop medical science from making advances that allow such choices or allowing the government to control the choices we can make as citizens. There are many important questions to ponder, including how to find creative ways to reduce or avoid negative consequences from even more dangerous alternatives. Consider, for example, what would our world be like if there were substantially more men than women? What if only the rich or only those who live in "rich countries" were able to choose the sex of their children? Is it likely that an approximately equal number of boys and girls would be or could be selected? If not, could a society or should a society make equal numbers of girls and boys a goal?
I am guessing that many readers of child-bearing age want to choose the sex of their (as yet) unconceived children and can reason that there is no harm in this practice. And, if you could also choose intelligence, height, and hair color, would you add that too? But then, there are few things in life that are as appealing as the possibility of a perfectly balanced family, which according to the modal response means an older son and younger daughter, looking just like an improved version of you.