You know what I think? I think they're looking to get married...to the next guy that comes along. Women get desperate to get married, especially between age 23 and 28, because all their friends are doing it. So once a bunch of her friends start getting engaged, and she sees all the ATTENTION given to her friends at these huge lavish weddings, she wants it for herself.
So the next guy who comes along...BAM!
The thing is, if YOU are the next guy to come along, you somehow resist the marriage impulse. She WOULD marry you if you'd chump up enough to give into her irrational pair-bonding desire. Of course, shes' more interested in marriage than in YOU, and in a lot of cases it'll just lead to common misery and apathy if you give into it.
But because you DON'T, she dumps you and moves on to the next guy.
And I think a lot of people SERIOUSLY underestimate the number of MEN out there who are under social pressure to get married. Sure, most men ACT macho, but there are a LOT of them out there who just want to hook up with a girl...because they believe they're not good enough to go after what they WANT and had better hook up when they have the chance.
In short...I wouldn't worry too much about it.