Traditionally men view the introduction of children to be a big step.
Some months ago I was using a dating site for single parents and was surprised by the number of men on there who are not actually single dads, or dads at all, for that matter.
A long time ago, it was hard for a woman to get a date, and it was easy for a man to get a date, and women with kids were seen as 100% committed to their kids, and not capable of sustaining a relationship. Plus, what with single mothers going to gyms or step class, you get a lot of very fit single mothers.
So single mothers are often just as attractive as single women, and in some cases more so.
The cynic in me would say that they figure since the woman has a kid she would be more willing to sleep with the guy.
But it could be that they can't have kids of their own and would ultimately like to join in someone else's family. But that would have been a LOT of infertile men in their mid to late twenties...