It's a social issue because there are social rules about how it works. It's not a government issue because they aren't rules people should (or do) get punished for breaking. Also, because it's nigh impossible for the government to control it anyway. They can refrain from calling you married, but they can't really keep you from living together, having sex, and possibly deciding to have kids. Not everyone that lives together is in a relationship. The government isn't going to know if you have sex. They could take away your kids and/or force you to get an abortion, but they won't.
If I die intestate, is who inherits my house a government issue?
If my husband wants to visit me in the hospital after my stroke, and the hospital staff refuse to allow it, is that a government issue?
Certainly, my government is currently involved in passing laws about such things. And whether I'm married or not affects how those rules apply to me. That seems to indicate that right now, marriage in my country is a government issue.
Perhaps if the government was not involved in any issues like that, to which the social status of my husband and me were relevant...
A thought occurred to me today as I skimmed an article in a rationality forum where the subject of gay marriage cropped up; seeing as the issue has been hotly contested in various public fora and especially the courts, what about poly? After all, many if not all the arguments for gay marriage apply to poly marriage as well.
Questions for LWers who are currently in a such a relationship, or have an opinion to share:
Do polies want to marry each other or do such relationships not lend themselves to permanence above a threshold of partners? Should polies campaign for the right for a civil union anyway? what are the up and down sides of this? etc