Your second point, RE: the continuous spectra idea, reminds me a fair amount of sexual orientation. When you think about it, like you and the author said, many, even most people in the west are still grappling with the mere idea of homosexuality. Suggesting bisexuality to even a moderately conservative person brings out anything from mild confusion to hostility. It's going to be a long road to convince the average person that there's a rainbow of sexuality, and everyone fits on a slightly different wavelength. But it's not because they're stupid - like you say, it's because there's a probably natural propensity for humans to organize things into neat patterns and discrete points, and have a hard time with fuzzy gradients. Perhaps its related to the ability to think abstractly? A strong abstract thinker can consider someone who isn't 100% heterosexual but has purely hetero relations, while a weaker one might only see the person purely in terms of their sexual actions and not their hypothetical place on the sexual spectrum.

Excuse the digression, there's always a lot of overlap between the ideas of sex and orientation! Very interesting.