You have to take into account how someone is raised
Everybody's parents/guardians don't raise them to be 100% for commitment or marriage
Then you have to take into account how them witnessing failed marriages, failed relationships or seeing staggering marriage/divorce statistics have affected their mentality about committment/marriage
There's so much that goes into it that shapes someone's fundamental ideals about commitment/marriage: media, personal experience (some have already went that route before they met you & b/c it failed or wasn't as fulfilling as they thought, decided not to believe in it anymore), what they've been taught by their peers, what they've been taught by their guardians, maturity/age level, etc.
^^^^ This..reading this and thinking about it. My mother has done exactly what I have replicated. One dad 4 children and one long term live in partner, no marriage.
Although one of my brothers won't marry, he has been with his gf for 15 years and they have one child. The other two are in marriages and still together after numerous years and children.
@Capgirl. Don't they now live in a de-facto relationship which means unless there is a pre-nup he is already entitled to 50% of everything! Works that way in this country and my homeland whether its a gay or hetro relationship.