Robert P. George explains why the Constitution doesnt settle the marriage debate :
A key question, perhaps the key question, this Court is being called on to address is whether the Constitution of the United States chooses between competing moral understandings of the nature, value, and social purposes of marriage, thus settling the question of how marriage is to be defined. On reflection, I believe your honor will see that it does not. Rather, the Constitution leaves the matter, as it leaves most matters of substantive law where choices between competing moral understandings must be made, for resolution in the forums of democratic deliberation and decision-making, including, in the case of federal law, the Congress of the United States.