Is it selfish to choose not to have kids?

Strip away the feelings and look at the incentives, because this is a textbook free-rider problem. Pay-as-you-go pension systems, public healthcare, and the general tax base all depend on future workers existing. Someone who opts out of producing those workers while fully expecting to collect the benefits is doing exactly what economists mean by free-riding: enjoying the commons without restocking it. You can call that a personal choice, and legally it is, but the math doesn't care about your framing. Every childfree retiree drawing a pension is being subsidized by somebody else's kids. If everyone made the same personal choice, the system collapses in one generation. A choice that's only sustainable when most people don't make it is, by definition, an extractive one.