It is hard to imagine how the government can determine legit alternative marriages from benefit scams. The reality is the more alternative arrangements we legalize, the more such scams will proliferate. Over time marriage will just mean less and not count for much. It's already happening within the heterosexual community regardless of gay marriage.
I probably sound like a grumpy grandpa, but I just don't think this bodes well for the future.
And get off my lawn.