You can't take parenthood back. Once you have a child, that's it, you've got one forever, it's your responsibility, your time, your money, and hopefully your love that's going to them. That's a lot of work, you are raising another living, breathing, thinking, feeling human being that is mostly helpless without you until they grow.
A child is not like a doll. Some people are unhappy with kids, some people don't love their kids, some people just can't deal with it. If somebody does not want children for any reason, they shouldn't have them, that could end up being harmful for not only themselves, but also their partner, and the child/children they have. Lives are not something to be toyed with, in my opinion you should only really have children if you really want them, and are capable of caring for them.
I don't hate kids, I like them, in small bursts anyway, but I'm not willing to potentially throw my own life away, and potentially bring some kid into this world when there's a good chance I could screw their lives up too. As for people who want kids but can't have any...I feel for them, I really do, that's really unfortunate, hopefully they can adopt or find some other way to make their dreams come true, but it is not my responsibility to raise a child just because they can't. I don't have to join a professional sports team just because a person who can't walk wants to but isn't able to.