1) I'm a woman married to another woman. Yes, one of us could get a donor, but it's one more hurdle, and it means we won't ever have one accidentally.
2) Debt. We've got some debt that needs to be paid off, and babies are expensive.
3) We'd need a bigger home. Right now the house we're in is paid off in full, and we'd have to go through the whole home-selling/buying process if we wanted enough space for kids.
4) Health concerns. I'm depressed, diabetic, and have fibromyalgia. My doctor says I also likely have PCOS and due to that I might not even be fertile. So I'm not a good candidate to try to get pregnant. My wife has her own health concerns, which are genetic, and she doesn't want to pass them down to a kid. I don't want to pass on the fibro or diabetes either. They both suck.
5) We're happy as is. We've got cats instead of kids (insert lesbians-with-cats joke here?). A lot of our friends have kids, so we're both pretty content to just be "aunties" to all our friends' kids.
Plenty of people do. For many different reasons.
Some people do not like kids, some people like their life as it is, some people are in a relationship that they are happy in but that they are sure of that it won't survive having children, some people carry genes that they do not wish to pass on and so on. The list is endless, probably at least as long as the list of reasons why people do want children, possibly even longer.
At the end of the day, the kids go home with their own parents.
How many people here would say that they have actively decided not to have children? I am just curious.