I really hate to wade back into this. First off, I'm not against gay marriage, only court-mandated gay marriage. Second, as mentioned before this subject has been beaten to death here at the RKMBS.

However....

Once you start making the argument that marriage is "right" or "privilege" that is protected by the Constitution and, therefore, can't normally be abridged or regulated, you create legal precedent that binds future decisions by the courts in other areas. When that happens, there is no way to create that precedent without, logically, also opening the door for such things as: (a) marriage between siblings; (b) polygamy.

In each case, you have consenting adults entering into a legal relationship that is at least as committed as a "traditional" marriage. Therefore, just as Snarf asks with gay marriage, a proponent of incest or polygamy can ask "what's the problem?"