The current ‘marriage equality’ debate, is really a debate about a redefining of what marriage is. From a Christian perspective there is no debate as the Bible is clear on the issue and so for Christians there is no change no matter what may or may not happen around us. What happens in the world happens there and that is not something the church has a say over in real terms. Certainly God does have something to say about it and he has said it through the Scriptures to the world today. Whatever happens in that world outside of Christianity, the Christian definition of marriage will never change, regardless of the pressure that may or may not be brought to bear upon it and/or the church of Christ.
It would seem to me that the next logical step – the next progression for relationships in the civil marriage/relationship space, but not necessarily with those seeking same-sex civil marriage legality, would be the polyamory culture that appears to be growing out there in the world.