https://www.pcmag.com/news/365330/new-facebook-policy-sparks-fears-of-sex-talk-crackdown
it’s not just Tumblr, apparently FB is looking into banning any chat of sex, including slang terms.
It was probably a lot of women who wanted to look at smut on Tumblr because they didn’t want the stigma of actually visiting a “porn” website.