Fun thread. I may have missed a few points in my skimming. So, apologies for what I've missed.
First, I noticed some folks talked about comparing sex to gender. After all, there is this culturally induced expectation that everyone is either a man or a woman, but that turns out not to be so. Once we recognize that, we have to consider not only masculine women and feminine men, but all kinds of other options.
In addition to XX women and XY men (the norms), there are XXY and XXXY among males and XO among females. It turns out that there are also all kinds of phenotypic traits that have to be considered, some of which lead to things like early gender reassignment surgeries.
But, that's just one avenue to pursue. There is also the more obviously biological. Nature sets a traditional (i.e. XX) female's threshold for responding to infant cues very low. The breasts of lactating women, for example, will sometimes spontaneously release milk even before the woman consciously registers the whimpering of her infant (or, on some occasions, another--even unrelated --infant). In traditional (i.e. XY) men, the threshold is set very high. Unless the man's brain registers that an infant, which he knows is his, or a female, which he knows shares a child with him, is in trouble, the hormones responsible for prodding him into action may not even kick in. (Interesting tests on some nonhuman primates, using mimicry, have been fundamental in helping establish this, and then later tests reveal some aspects of how this works in other nonhuman primates and in us.)
There are good reasons for nature tweaking our responses in this way, most of them having to do with the success of our ancestors (where success is defined in terms of how many grandchildren one leaves behind by the time of one's death, and more deeply defined as the total number of one's decendants). Our ancestral mothers who didn't respond quickly to their infant's needs sometimes had those infants taken by cats or snakes or other predators. Our ancestral fathers who responded to quickly might have been "out-sired" by other males who ignored most calls in favor of seeking more females to attempt to impregnate.
Anyway, thought I'd throw this into the mix of an interesting debate.
Dave