In a way, some aspects would be much simpler if marriage wasn't entwined with the legal recognition of the Government. But there'd be a lot of things left open ended if marriage wasn't legally relevant. I feel it would be a lot more unfair of a setup if we didnt recognize it.
If we didn't recognize marriage in a legal way:
1.) When a spouse dies, their family could potentially inherit almost all of the couple's assets (considering people's in-laws don't always like them). So in other words, well your love died, now you have 30 days to pack your stuff and get out, because your father/mother in law is a piece of shit.
2.) The blood relatives can decide to take a person off of life support, possibly if it goes against their and/or their spouse's wishes. [Skip two spaces, go back to point #1 above].
Now the case that the idea of marriage itself has flaws and shouldn't exist is an argument I could potentially get behind, from a social science standpoint.