Very true: My marriage is legal. We didn't actually care if we were married or not, but boy oh boy immigration would rather us be married. So we are. We'd be together nonetheless, marriage was just the means of doing so.

But I think the poster's point was that this advice is basically advice for long-term relationships and really shouldn't be viewed under the light of marriage only. Since many folks don't have to be married to be a family, a good number of folks are skipping that step.