I'm just cynical maybe, but I can't help but think if you love each other enough, who needs a bit of paper to keep you together? If not, 10 years down the line, why should that piece of paper keep you miserable?
We're mainly out of the societal pressure to stay attached to one person for 80 years. My way of thinking doesn't stop that, just stops the unhappiness of people who are trapped by it.
Of course, I say that, my friend got married, and now has a kid and I think it's possibly the best thing he could have done.
That's buggered my theory.