My gripe is that marriage, as an institution, ought to evolve to meet the people's changing needs, and by codifying it, the government locks in features (like the assumptions about gender or money or about who gets to make medical choices of certain kinds) which might then later fail to match up with how the people are living.
So it's not that I think it's irrelevant, it's just that I think it could serve us better if we kept a cleaner divide between what we practice socially and what we legislate. Whatever it is that you might need a reference to marriage for in the law, instead create separate decoupled systems for handling those things. More flexible that way.
We weren't going to bother with the marriage paperwork, because who cares what the state thinks. But then we did our taxes both ways and decided to swallow our pride and take the money.