I do not want, have zero (negative if there is such a thing) desire to, and have been around children from birth to adult leaving no mystery to what life is like with kids. My home situation growing up put me right in the center of raising my siblings, and all I wanted was some freedom.
Add in things like the simple fact that genetically I shouldn't be reproducing - there are enough genetic faults that were passed on to me. I don't like children, even in small doses. I have dangerous hobbies, and am not willing to give them up for anyone else - and that's not fair to a kid. I saw what happened to my mother, aunt and cousins' bodies after having children - and don't want to do that to mine (genetics rule here).
I'm sure if it was forced on me (say, something happened to my mother and I had to take in my nephew) that I'd do a perfectly good job of it, but it's not a job I'm going to volunteer for.