I'm not having children. There are a lot of reasons.
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.