Well, I’m a single gay man, so while it’s unlikely that I would impregnate someone by accident, I am, and expect to remain, childless by choice.
The reasons are simple.  Children, and the rearing thereof, require vast amounts of time, patience, energy, and money, none of which I have in adequate supply–not even for my *own * life.
Also, I’m 38 now, and I’m approximately seven years away from finishing my B.A.  Even if I wanted to wait until I graduated college to raise children…well, I hardly think that I’d want to be raising children into my 60’s.
Now, if someone trusted me enough to leave me their kid in a their will (does that even ever happen?), then I’d certainly man up and do what needed to be done for the child, and I think I’d do okay (my friends tell me that I’d make a great father).
But apart from that–or some other extreme circumstance, like if I knew of a child who desperately needed to be rescued from a bad situation, and I felt that the best place for them to be was with me–I don’t plan on having kids.
The thing is, though, I tend to get along well with most children.  Hell, they piss me off much, much less than most adults do!  And the fact is, I’m a nurturer, so I like to “mother” kids (especially boys since, IMO, boys don’t get nearly enough “mothering” from males in this society), but nah, raising children is something that I am, within 99 44/100th percent certainty, not likely to do.
P.S.–Like Annie-Xmas, I’m a survivor of an abusive dhildhood, so I totally understand her reasoning in that regard (I don’t see my brother having children for the same reason)–and, frankly, applaud her for taking it into consideration.  For me, though, I think that my experience with taking care of other people’s children (I know–it’s not the same as full-time parenting) has shown me that, thankfully, the abuse I suffered has taught me what kind of care-giver NOT to be.  Of course, that doesn’t work out the same for everyone, and sometimes you really just don’t know how you’ll handle things.  I wish that my idiot parents (especially my father) had kept that in mind.