One consideration in evaluating the ethics of sex selection involves the timing of the selection itself. If a boy is desired and the sex is to be determined after fertilization, a negative result (meaning the birth of a girl) can only be prevented by killing the embryo or fetus either in the lab (if conception is achieved through in vitro fertilization) or in the womb of the mother (by abortion). Such sex selection should be opposed for two reasons. First, an innocent life is sacrificed. To terminate a life simply because it is not the "product" one desires is demeaning and discriminatory. Each human being, male and female, is uniquely created, known and valued by God (consider Job 31:15; Ps. 139:13-16; Isa. 49:1; Jer. 1:5; Gal. 1:15; Eph. 1:3-4). All human life exists primarily for God's pleasure and purposes, not ours (Col. 1:16). The sanctity of human life, created by God in God's image (Gen. 1:27, 9:6), demands that a new innocent life not be ended for any reason except possibly when the sanctity of human life requires it - i.e., in the very rare case when doing so is the only way to prevent that life from causing the death of another (the mother). Second, even in a country where abortion is technically legal, one would hope that restrictions would be put in place to prevent the taking of a life simply because that life is the "wrong" sex. It is not any child's fault that he or she is conceived as a boy or a girl, and there is nothing inherently wrong with being either.
Another way to determine the sex of a child is to do so before fertilization. Procedures exist that can largely separate sperm that carry the Y chromosome from those that carry the X chromosome. Eggs fertilized by sperm carrying the Y will be male and eggs fertilized by sperm carrying the X will be female (X plus Y produces a boy; X plus X produces a girl). If the sperm sample used to fertilize an egg in the lab has been selected for the Y chromosome, the odds are greatly increased (about a 90% likelihood) that a boy will result rather than a girl. In line with the previous paragraph, it is essential that anyone using this method be willing to accept either a boy or girl and not discard the embryo or abort the baby after the sex is confirmed.
However, is the use of this method a good idea in the first place? One proposed reason to utilize this procedure is to help insure that a child will be free of a sex-linked genetic disease. Red or green color-blindness, hemophilia A and B, and fragile X syndrome are all due to mutations on the X chromosome. Therefore, males are much more likely to suffer from these conditions (in females, the second X chromosome carries the normal gene, and the presence of the mutated gene on the other X chromosome may have no effect). Where there is a known risk of one or more of these diseases in a family, selecting for a girl by sperm selection can greatly reduce the possibility of having a child with any of them.
The motive of reducing suffering is laudable, particularly when suffering can be reduced without the forfeiture of a life. Nonetheless, sex determination by sperm selection is not wise. It seems innocent enough for a couple who already have a boy to simply want a girl to balance their family. But why is this so important? What fuels this desire? It is dangerous to think that we know more about life and God's purposes than God Himself. The idea that we can control our destinies and that of our children is arrogant. There is a purpose for each child, boy and girl, that is far beyond our human capacity to grasp (Isa. 49:1; Jer. 1:5). In some cultures where one sex (usually male) is preferred over the other, widespread sex selection could easily lead to a dangerous imbalance between the sexes, threatening the future social stability of the culture. Furthermore, choosing one sex over another is another form of discrimination that does not need to exist in a world where discrimination of all other sorts has already caused great havoc.
Even when the purpose of sex selection may be to avoid a sex-linked disease, we run the risk of communicating to those who have uncorrected or uncorrectable genetic diseases that they would have been better off not being born. Though no disease is wanted, those who carry them are. And may we never hear in our technically advancing society parents saying to their child, "I knew we should have opted for a girl when we had the chance." Such emotional abuse is inevitable in a society where technology is intended to give us what we want, but where human nature still leads children (both boys and girls) into paths no parent wants.
Imposing characteristics on children that all human beings would welcome is controversial enough. Imposing on children without their consent a characteristic like maleness - which not all human beings would want - is the first step down a road that we should not travel. It commits us, in principle, to allowing parents or society to impose on a child whatever characteristics they wish, regardless of whether or not such characteristics are what that child would want.
Ray Bohlin, "Is It Ethical to Determine the Sex of a Child?,” Dignity 7, no. 3 (2001): 4.