he rattled off a ton of examples of things in humans and other species that don't fit easily into the simple male/female dichotomy.
then he summed it up brilliantly: nature just does what it does, in all its wonderful diversity, and then humans come along and try to understand it, making simplifications because we can't readily comprehend all the ways in which every individual life form is different, so we create simple boxes to understand MOST, but not all, of life.
but nature doesn't care about or observe human's simplistic models. it just does what it does, leaving humans to keep improving our understandings to better reflect reality.
a few things he mentioned:
humans aren't just XX or XY, there's also X, XXX, and XXY.
some species have both male and female parts (flowers have both stamen and pistil!)
some species can change sex (some frog species -- if there are "too many" of one sex, some will simply change to the opposite sex!)
homosexuality and bisexuality is in many species as well.
all fetuses start development as female, and many things have to happen in order for a baby to be born expressing maleness. it isn't just a matter of having XY chromosomes. for various reasons, a fetus may not produce enough, or any, testosterone. alternatively, the receptors may not process the testosterone, at least in certain organs. so the fetus just continues to express femaleness, notwithstanding the XY chromosomes.
then there are also chimeras -- an embryo can absorb a twin early in development. sometimes, one is XX and the other is XY, which can result in some organs being female/XX and others male/XY in the same person.
again, nature does what it does, and humans struggle to comprehend. in the process, we make simplifications, and idiots get upset when confronted with the reality that their simplification doesn't handle 100% of all the cases.
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