I do believe it is when they fall in love. I think men are more likely to keep "light" relationship going until they find THE ONE. Usually women don't see that he is not as commited as she is until it's too late. This is why we women call them commitment-phobes. There was very popular book out 2/3 years ago titled "He's just not that into you" . It got a lot of flack, but was very enlightening. We help men keep these fake relationships going by tolerating booty calls, lame excuses about why he did not call/come over and call them relationships when his every action or inaction should scream to us that "HE'S JUST NOT THAT INTO YOU"
As the author very clearly states, whe he is into you he will call, see you, and do the right things. This is when they settle down. IMHO