In a world that still suffers from gender discrimination, allowing parents to choose the gender of their children opens a new frontier for discrimination to exercise itself. In China, a one child policy has lead to an excess of males, regarded by families as more desirable. Even if people try to refrain from discrimination, at a large scale even the slightest subconscious biases can materialize as broad societal imbalances. These can have implications for a broad range of gender issues. What's worse, gender imbalances may affect public heath in unexpected ways. No one is as neutral as nature, and no one is better able to look after the long term well being of the species than evolution.
Pre-selecting the sex of a child is the start of genetic engineering. This cannot be allowed for moral reasons. The next thing you know parents will be selected desired attributes of their child. This will lead to social issues that we cannot even begin to understand. This should not be allowed.