Well, natural phenomena (like having mates and children) don't always go according to the plan.[1] That's why nature populates the earth with enough people to ensure most of them will follow the rules of evolution/biology/whatever.

But I'm guessing that even those people who don't want children are following the rules of natural selection. "Only the fittest will survive" is then translated to something like "only the genes of those who want to pass on their genomic data will survive".

[1] People have free will after all.