I believe the problem is never showing evidence, but that the evidence is overwhelming. I could explain the general idea and, maybe, one or two specifics. People that use the XX/XY binary argument wouldn’t be able to explain either, but it’s usually only used because it conforms to a bias. And we are only talking about humans here. Language would implode if we tried to maintain convenient binaries and still back it up with science.
This is a nice graph for debunking the idea that biology offers a sort of refuge for the proponents of a strict binary sexual framework. Let it be known that once in a while some people born with penises are XX while some people born with vulvas are XY, and that this is just the tip of the iceberg. Maleness and femaleness might be fact as poles of a spectrum (might be more complicated); what they are NOT is an either/or mutually exclusive phenomenon. The distribution of the population on that supposed spectrum probably look like an inverted bell curve, with most persons closer to either end… but to be honest I suspect the curve is not as pronounced as we are led to think.
Personally speaking I’m forty something and by now I’m convinced I’m not 100% pure male stuff. I don’t give a shit if it’s biological or social. I feel it in a myriad of ways. I’m comfortable enough in the way I’m treated and perceived though so I let sleeping dogs lie and ride the male label. I’m hairy, I have a dick, I don’t want trouble. So this is totally unscientific, but I swear sometimes I recognize this same ambiguous essence in someone else; there’s a faint feeling of kinship. I’m willing to bet the silent majority has plenty of folks who wouldn’t have minded being a wee bit further from the poles if others didn’t make such a fucking fuss about it.