The last time I was reading on this topic, around 10% of kids are actually unwanted -- that is, one or both parents did not want to ever have kids, or did not want to have more kids. Most kids are planned to some degree. For the remainder that aren't, the couple does want to have kids at some point, but they didn't plan to have kids at that particular time.

Regardless, children are optional, full stop. There are certainly reasons why people who don't plan to have kids end up having them anyway, but that doesn't change the fact that it's an optional activity.