No, pre-selecting sex of children is not a good way to avoid diseases. While I do not know much about the medical field, I think that pre-selecting sex is morally wrong. Given the society we live in, this technology will be used to favor one sex over the other. The way people are made is the way they are meant to be made.
What diseases are we trying to prevent by choosing the sex of a child? Since the question is not really complete I'll give you my opinion on choosing the sex of a child at all. IT'S A TERRIBLE IDEA. It would likely lead to many males and few females with many people trying to pass on their lineage to a male heir. It would make the remaining females left more sought after, but likely leading women to become an underclass again!