Incestuous marriages will still not be allowed, even though every argument for allowing two men to marry applies equally to allowing a man to marry his mother. Same thing with polygamy. The incest and polygamy proponents just don't have nearly the political pull as the homosexuals.
Incest may not be allowed now, but the basic purpose of marriage as we know it today (knew it before this Mass. thing got started) is to bring children into the world and provide a stable environment for raising them.
A same-sex marriage does not allow for the two people to have their own child naturally. So, once marriage becomes about the parents and not the children, all the old rules can be eliminated. If you're not going to have children, who cares if you marry your sister, or parent, or child?
I'm not planning anything like that myself, but I can see this as a primary unintended consequence. And wait until you can marry your clone, so you can pass along your wealth with lower taxes....