As a 21 year old guy I have gone back and forth on this topic. Originally I would have said, no, kids were a deal breaker. I than begin to wonder if perhaps I was giving up the chance to me with a great person because she might have children from a previous relationship. So my current philosophy is that her having children won't stop me from communicating with her or perhaps going out wit her and seeing if there is chemistry.
Now it would be a definite no if her desire was to find a father for the baby, as some women are indeed looking for that. While if the relationship worked out I would gladly accept the duties of a stepdad if the situation was to arise; however, if there was any indication that she was looking for that right away I would likely be very turned off by it.
You basically explained what I was tried to explain, but made it much more understandable. Like I said I am not looking for a father for her, but if the relationship progressed to marriage I would expect him to take on the role of stepfather. Nicely put, thank you. I am wanting a serious relationship, but it would be quite sometime before the two where even introduced because I do not want to bring people in and out of her life, and I would not want to introduce them with the impression that she was now his responsibility also.