The "obvious evidence to the contrary" was the math gender gap, which was very real at the time. If you actually read the story, it's quite clear that Feynman is telling the story at his own expense (showing how wrong he had been about girls + math). There's no question he was sexist, but see _debug_ and blackguardx's tandem quote to see that he regarded them as equals mentally: http://news.ycombinator.com/item?id=1197382