I'll be honest, I raise an eyebrow a bit at the whole "marriage is different because it's a commitment for life" thing, because for most people that isn't really something they believe. It may be a hope and aspiration for married couples, but it's also a hope and aspiration for unmarried couples. Ultimately though most people would divorce under the same conditions unmarried couples would separate - infidelity, abuse, falling out of love, falling in love with someone else etc etc. I wonder how many people claiming they are committed for life would still choose to marry if they knew that divorce wasn't an option?
I've only known 1 person who I genuinely believed was committed to her marriage for life and she was an extremely strict Catholic. But even she still has the legal right to end her marriage if she chooses to, and she can end her relationship even sooner without ever needing to end the marriage. Her commitment comes from her beliefs, not her actual marriage contract.
My friends boyfriend, who she lives with and is pregnant by, has been separated from his wife for 3 years and seems to have no interest in getting divorced. It's not a great situation but the fact of him being married clearly means absolutely sod all in terms of a lifetime commitment.
The idea of marriage for life is sweet, but in reality most people are just married until they don't want to be any more, and the point of the marriage is just to define the terms under which they may one day separate.