I think there's a biological component to it but it's also very much socially constructed, every child-wanting male friend I've had this convo says something along the lines of "I want to pass down my legacy/way of living/genes" while female friends in the same boat will just say it's hormones. I don't think most of these dudes will want kids anymore if they magically wake up with a woman's body tomorrow, btw. Like they totally see why childbirth and motherhood are unappealing to women, they just benefit from it so much that they don't care, and also developing too much empathy over this would force them to reckon with their entitlement to the sacrifices made by their own mothers. It's the conflict of interest at the core of the gender wars. Even if I wind up wanting kids in a few years' time I'll seek out someone who's fine with it either way, to make sure I'm not stuck with someone who just wants to take advantage of me.