In one of the group homes I lived in after college (not like it sounds, but too haphazard to be a commune) one of my housemates had placed in the bathroom an oversized children's paperback book called What Is a Girl? What Is a Boy?, affixing to it a note that said something like "this is for all you losers who can't tell the difference." The book was a photoessay showing boys and girls engaged in all kinds of anti-gendered behavior, and the last two spreads showed a naked boy and girl, then a naked man and woman, explaining that genitalia was the only meaningful difference between the sexes. By Stephanie Waxman, it was published in 1976 by Peace Press in California . (It was republished in 1989 by T.Y. Crowell. Had Us become Them?)
K. T. Horning's post "Retro Reads: Before Heather" makes me remember those days. I'd love to have the Horn Book take a good look at this era of leftist small press publishing for children--any takers?