It’s not Tumblr who is making the decision. They’re complying with the new law regarding Sex trafficking. “Sex workers” posting imagery is regulated under the new law.
https://en.m.wikipedia.org/wiki/Stop_Enabling_Sex_Traffickers_Act
Whomever came up with the term “female presenting nipples” needs to die a hideous death.
I am neither pro or con on Tumblr’s decision, but you are wrong as to its cause.
The Tumblr decision goes far beyond anything needed to comply the sex-trafficking law.
“Adult content” is not necessarily in and of itself part of “sex trafficking” or contributing to it.
The legal answer has to find a sex trafficking context in either who puts the material up, the persons depicted in the material, or any suggestion of sex trafficking the material makes. That is not to say much adult material is not offensive to a good many people, but only a reminder that not all such adult material bears a sex trafficking relationship. Tumblr’s decsion goes far beyond the legal requirements of the sex trafficking law.