NHK, Japan's national TV broadcaster, will be showing an English version of "Our Comiket's History: 50 Years", a 45 minute special covering the history and present of Comic Market.
Watch it live online April 26 10:10/16:10/22:10/04:10 JST or later from the archives (link below)
If we assume that "bishonen manga" means a certain taste and styles of the '70s shoujo manga creators, it can be traced back to Okada Fumiko's late '60s experimental manga.
"Otokonoko" appnared in the early '70s shoujo manga, so there is not much of time difference.
I just realized that this shot in the Zan Sayonara Zetsubou-Sensei OP is a reference to this poster for Shuji Terayama's "Throw Away Your Books, Rally in the Streets".