I assume we are talking about sex selection in humans. One obvious point in favor is that people would be able to get a boy if they wanted a boy and a girl if they wanted a girl. Another point in favor might be that there would be a better balance of siblings in a family. This could benefit the children, because there would probably be less sibling rivalry between, say, a brother and sister than between two brothers or two sisters. Furthermore, there might be some diminution of population growth. Some couples who have, say, two boys will try a third time hoping to have a girl, whereas they might stop with two children if they had a girl and a boy. Same, of course, if they had two girls and wanted a boy. (And some couples have five girls or five boys just hoping to have one of the opposite sex.)
One point against sex selection is that it could lead to an imbalance of the sexes within a population. This has happened in China, where the government has tried to reduce the population by permitting a woman to have only one child. The Chinese have traditionally favored boys for economic reasons, and there are now more than 25-million males who will be unable to find wives because of the imbalance. Another point against sex selection is that it almost inevitably leads to abortions. In China and India, as well as elsewhere in the world, it is now easy to determine the sex of an embryo, and many women are having abortions of perfectly healthy infants if they are not of the desired sex. This sort of interference with nature seems not only dangerous but a waste of medical expertise on what seems like a frivolous fancy. After all, having a healthy baby is what is important. Discrimination against infants on the basis of gender is as immoral as any other kind of discrimination and should not be encouraged.