This seems like the obvious explanation, though I think your use of the term "hedonism" is distracting. People are inherently selfish (how could any entity not be "self"-ish to some degree?). The bottom line is that we do things because we want to. Even selfless activities feel good. That's fine, honestly. But having children is not intrinsically non-hedonistic. It's just one of many self-fulfilling activities we choose from.

Agreed with everything you wrote. I meant “hedonism” in its non-colloquial, neutral sense, i.e. the pursuit of individual pleasure and happiness above all else, which judging by the other replies was a mistake on my part.

>But having children is not intrinsically non-hedonistic. It's just one of many self-fulfilling activities we choose from

Exactly, and my point is that all the activities I listed (which only become abundantly available in rich, industrialized societies) are more self-fulfilling than having children.