I've always thought that buying a property together or having a child was a far bigger, more important commitment than getting married. And obviously, loads of people have long, successful relationships without being married, when neither partner is interested in marriage. BUT there seems to be so many people who have a child together and buy a house together, and then one of them wants to get married but the other drags their feet and they eventually decide they don't want to be together after all. It seems like marriage is the only commitment where you are actually committing to each other, not to a child or to a financial investment, and so maybe marriage is quite a good proof that both partners are committed to staying together.
I know this ignores the fact that a few people get married knowing its the wrong thing to do, and people do change and marriages break up. I'm just saying, I've only just realised that maybe I was wrong to think that children and property are proof of a commitment to the person you're doing them with.