Yup. There's no rational basis to treat married people any differently, unless we think that marriage is good for the fabric of society and want to encourage faithful monogamous couple and/or families. And if we want to encourage healthy families, then there is a rational basis for defining marriage as being between a man and a woman, the only combination that can have children. So I can arrive at either endpoint... special treatment for only man and woman, or no special treatment at all. The latter, I suppose, would be the more libertarian approach.