If we are going to go the way of same sex marriage--and I have no doubt that we will- the question becomes, where do we draw the line. If the case for gay marriage can be made then the case for multiple marriage partners cannot be denied. If scientist can "evolve" and testify that incest is not necessarily a harmful thing then the case can be made to support that also. I know that that sounds implausible and farfetched but then what would be there to stop it?
You want equality? OK. Fine. What are the unintended consequences?