I had to disagree with his article, because there is a fundamental difference. The incest case was about two people who got married illegally. So, despite the author's claim that they are similar, they aren't. The cited case specifically stated it was not dealing with whether the couple could get married, nor about whether the government had to acknowledge it. It was in fact simply whether two adults could do what they wanted in private.
And it was because of this that the judges couldn't find any legitimate government function which would justify the prohibition. They couldn't find anything in the record about how the government was effected.
In the incest case, the couple was married, which meant the government had been involved, and therefore could say they had an interest in promoting only certain unions.
I don't disagree with Scalia's dissent in the cited case, but I think the author of this piece is a little out to lunch.
Still, the article does bring up a good point that if gay marriage is found to be constitutional, incest would be constitutional for the same reasons. (Although, that reasoning may not be quite the same as in Lawrence v. Texas.)