[William McKinley]

But didn't he say that if gays can have sex and not get arrested then incest (among others) should be 'legal' too?

No, that's not what he said.

What he was talking about is the legal foundation for such laws. He said nothing about what should or should not be legal.

If the Supreme Court finds that there exists a right to privacy in the home that means that sodomy laws must be thrown out, then the same ruling would mean the end of all those other laws, since they have the same legal foundation.

Obviously, he thinks that would be a very bad thing, which is why he is making the argument that striking down the legal foundation for the sodomy laws would be a very bad thing.

You might want to read this article if you haven't, because the author actually examines the very point Santorum is making, and concedes that Santorum's point is not easily answered.

[Laurie S]

Here's what he said: "If the Supreme Court says that you have the right to consensual [gay] sex within your home, then you have the right to bigamy, you have the right to polygamy, you have the right to incest, you have the right to adultery. You have the right to anything," Santorum said, according to the AP

It sounds to me like he is saying that those things are now illegal and if the supreme court declares that consensual gay sex within your home is considered legal, then all those other things will be legal too (as if they are not now legal--some are actually illegal, but I didn't think incest was technically illegal, as in, one doesn't now have "the right to" commit it.