Your last post was full of stuff I agree with, which is a refreshing turn.
Foremost, I have absolutely do doubt that should some future case involving incest or polygamy come around in Massachusetts, they will use what they can from this current ruling. They will use whatever they can. And it will be up to the judges of course, to discern the differences as it regards the MA constitution.
In terms of civil law, I'm not sure what exactly the incest side will be fighting for as many of the rights and privileges gay couples are looking for are their for family members. But I'm sure there will be something. And I agree that the genetic argument may be mute for the reasons you stated.
Disturbingly, if you have to take the genetic issue out of the argument (a thorny thing, that) it actually has me wondering if the con arguments would be strong enough to hold. It is a thoroughly creepy thought to me, but I'm aware of course, that homosexuality is no less creepy to many others. I myself try not to dwell on the mechanics of it much.
So does the slippery slope exist? Sure. Does it portend inevitability? No, but I certainly recognize its presence.
Still, I for one, am not about to argue that committed gay couples (I keep using that term, awkward as it is, as I want to distinguish from the gay equivalent of shacking up) have to be denied rights and privileges of married couples because of a concern about polygamy and incest - issues I'm sure are just as foreign and unacceptable to them as a whole as they are to us.