As a teenager I always said I don't want kids at all, mainly because I didn't even consider adoption as a way to have kids. Nowadays I still don't feel the 'urge' to have a family but if I ever want kids I would definitely adopt. So many people don't care about protection or don't think about the hard time of raising a child, so they get rid of it and put it in an orphanage. I would rather give an already living being a chance to live a proper life than giving birth myself. Not only the birth can be painful (there's always C-section and if you're not afraid of scars, it's an option or maybe even the only way for ones to give birth), but also the pregnancy can be a difficult nine months.
I have a friend who always used to say "If you don't want kids, then what do you want?" .. because you know, giving birth is the only purpose of women. Not like there are women who couldn't give birth, even if they wanted to.
He's still my friend and gladly changed his opinion, even though he still thinks it's weird to not want own children.