"M&F seen as having different natures" "F->M direct child care" "M->F more competitive" "M->F greater spatial range" etc
Steven Pinker talks about this (and much else) in this speech. Skip to 26:56 for this particular point. https://www.youtube.com/watch?v=n691pLhQBkw
An objective truth doesn't have to apply to every individual without exception, to be an objective truth. It just has to be a consistent trend. So M/F differences like the above really are objective truths, the same as "men are taller than women" or "cougars are more dangerous than goats".
(edit: this is not to say anything prescriptive about what ought to be done; it's simply a descriptive statement. Of course Pinker covers all this ground; I recommend the whole talk)