same-sex marriage, polygamy, and incest

Over at Balkinization, Professor Balkin writes about the legal theories under which same-sex marriage could be declared legal. One of the theories is

The ban on same sex marriage violates a fundamental due process right to marry.

He calls this theory weak because of the difficulty in establishing a compelling state interest that allows same-sez marriage but disallows even more controversial practices such as polygamy and incest. This strikes me as a potentially strong political argument for finding a different legal basis, but a remarkably weak legal argument. If the arguments against legal polygamy and incest are indeed so weak that they aren't compelling, why should they be banned? If the arguments are grounded in mere prejudice and "tradition," shouldn't they be legal? Shouldn't the presumption in a free society be that conduct of free individuals should not be limited without good reason?

Suppose the right to same-sex marriage were affirmed on the basis of a fundamental right to marry, and suppose against all expectation that such a ruling were affirmed by the US Supreme Court to be the law of the land. According to the statistical abstract of the US, there are 925,000 working lawyers and judges in this country, and many more legal amateurs who'd be interested in making sure that polygamy and incest don't become legal. If none of them could find a compelling legal basis for the bans, what does that say about the reasons such practices are banned?