Maybe because some people are different and not always in ways that are good?
Maybe because children need to know that there are concepts like RIGHT and WRONG?
Maybe because the Obama children need to learn how to make value judgements so they don't grow up to be brain-dead liberals like their mother and father?
They are not being treated differently. I am not allowed to marry a man either. My wife can't divorce me and marry a woman. We are all being treated the same way.
The fact that I don't want to marry a man is irrelevant.