OK - I've been trying to stay off here but I can't resist. I'm not sure some of the posters appreciate just how hurtful it is to be told that the difficult situation you find yourself in is largely your own fault - if you had been braver, less like a pair of fluffy slippers, less concerned with your career or socio-economic status you'd now be happily married with children 
I'm 33, living with a partner but not engaged, no children and not much likelihood of them. I was in 2 long term relationships through uni and into the 1st year of my PhD, the last of those finished because among other things I wasn't sure about children and he definitely didn't want one. Through my PhD I had no success in meeting Mr Right, despite internet dating - a series of disasters. Then I started work, my social group was all new graduates 4 years younger than me and mainly out to party or in relationships since uni. So more internet dating, as before relationships of less than a year, waste of time.
Then I met my DP through work, he is 4 years younger than me. At this stage I still wasn't sure I wanted children, neither was he, all fine. After 18 months we moved across the world together and interlinked our careers and finances. I saw this as the 1st of a series of commitments. A year later no proposal has been forthcoming despite various discussions about weddings and so on - not unreasonable, we've been together less than 3 years. However, I've also become increasingly leaning towards wanting children so started to mention the idea. He's still not sure - he wants to wait until he's 30 to discuss, I'm acutely aware that I am then 33.5 and the clock is ticking.
I am happy with him, enjoy our relationship and our lifestyle here. If we were to split we'd have to move back to the UK, it would impact both our careers and finances and then as many on this thread show, there's no reason to think I'd be any more successful than I was in my presumably more attractive 20's.
So it's not as simple as 'he's a b*stard and won't commit' - he's not sure, as I wasn't at his age, but by the time he is my age it may be too late. It's also not as simple as 'dump him and get a 'good' man who wants to settle and have babies'.
Anyway, reading the 1st dozen pages of this last night reduced me to a sobbing heap on the sofa. DP asked what was wrong, which lead to a very open and heartfelt conversation (on my side) that resolved nothing. Probably need to start a thread of my own on relationships.