Because the state has a compelling interest in it. Because marriage and the family are the bedrock of society. Because the 10thA reserves to states rights not enumerated in the Constitution. Ergo states have the power to define marriage, not some unelected judge somewhere.
50% divorce rate is the bedrock of US society? Damn, absolutely screwed.
So now you're using the 10th amendment to make your point, and ignoring the 9th, and 14th, and the Supreme Court decisions on this? Well done.
50% divorce rate (and that's inaccurate) is irrelevant to the fact that most people who get married have children.
If there is a right to marry, then the power to define marriage rests with the states, not unelected judges. Something about they derive their legitimate power from the consent of the goverened.