but heterosexual marriage does not? How does gay marriage opening up the door for incest?

The 'slippery slope' right? That argument is a poor one that doesn't really stand up in any true forum of debate.

I leave you with this quote from a man more articulate than me:

"The precise challenge for morally serious people is to make rational distinctions between what is arbitrary and what is essential in important social institutions. ... If you want to argue that a lifetime of loving, faithful commitment between two women is equivalent to incest or child abuse, then please argue it. It would make for fascinating reading. But spare us this bizarre point that no new line can be drawn in access to marriage—or else everything is up for grabs and, before we know where we are, men will be marrying their dogs."
-Andrew Sullivan