> Didn't Child himself comment somewhere about the irrationality of being able to publish all that nasty stuff but unable to publish anything about normal healthy sex?

It have been more than exceptional had Child done so in the 1890s, or at any time during the Victoria era. And it's difficult for me to imagine him even thinking it.   

Consider his headnote to "The Keach in the Creel," a humorous ballad which he somehow forced himself to include, despite its including a passage that was "brutal and shameless almost beyond description."

Nowadays Mudcatters have argued over what the hell passage he could have meant!

No, the statement you allude to was made by the above-mentioned Legman in the early '60s - and frequently thereafter.