Government ISN'T defining what relationships you can form. This case is about what relationships the government recognizes as a long-term benefit to society. Government recognized marriages in the past in order to enforce/ensure rights of inheritance. This later expanded to protect the rights of wives and promote safe healthy environments for children. The homosexual agenda in this is simply trying to portray their lifestyle(s) as healthy and "normal". I find it a bit irritating to be spending so much of society's resources, time and attention trying to promote the cause of the most powerful (politically, socially, economically) minority (~1% of the population, perhaps 2% in recent years) in the history of homo sapiens -- or perhaps that should be homo sexualis given the fact this issue has less to do with thinking than with who we have sex with.
As I said in an earlier post, there are wider, deeper multicultural roots to support incestuous marriage and polygamy yet those are still sneered at by society while the cultural elites have a lot of sheeple bamboozled into protesting for homosexual marriage. Yeah, it's time to change the name of our genus ...