But that behaviour (monogamous, courting and marriage) can absolutely be seen as an evolutionary adaption. Male birds do not really need to help a female build

But that behaviour (monogamous, courting and marriage) can absolutely be seen as an evolutionary adaption. Male birds do not really need to help a female build a nest or look after eggs. But if they don't the female will deny access to sex, or the offspring will never hatch. And the female could just mate with any random bird, and not bother building a nest. But the egg won't survive. The same is true of humans. If a males want their kids to survive to reproductive age then looking after their mum and the kids during infancy is the best strategy. And the best strategy for the mum is to use what power she has to encourage that. That may not be entirely a reflection of biology now, but it certainly was in the past. And it is no accident that humans find certain traits attractive in a mate. But the difference in cost absolutely does still exist. Single parent households where the mum is left holding the baby are not exactly unusual. And the health burden of carrying a child and birthing is 100% on the women.