First, I have never dated anyone with kids, and now I am with someone who I plan to spend my life with, who does not have kids, so anything I say here is purely hypothetical.
Second, I don't understand why those with children get so hostile toward people who don't want to date someone with children. Everyone has certain qualities that they want and don't want in a partner. Not wanting to date some one with children is no different than not wanting to date someone because they're a workaholic, or because they're a slacker, or because they haven't been to college, or because they don't like to go to bars, or because they smoke, or a million other things people do. None of these traits make you a bad person, it just means that you won't be compatible with some people.
I, personally, would strongly prefer to date someone who does not have any children. I don't have kids and I don't want them. I am a bit selfish and I know it, and I am not ready to stop doing things the way that I want to do them. I don't want to be responsible for anything other than my cats and myself. Therefore, I don't think I could be a good step parent a good step parent and still be true to myself. I grew up with divorced parents and I hated the guy that my mom dated with a passion, and I'm sure part of my desire to not date someone with children stems from an unconscious desire to never want to be like him.
I won't say that I would never date anyone with kids, but I would have to be compatible with that person in just about everything else, for me to make that big of a change in my life. Then it would depend totally on the situation and the personalities of the person I was dating, and that of their child(ren).