Addendum about Saatchi - artwork is rated by the curators, but the category is wide open and only speaks of legal prohibitions (i.e. minors as nude photo models etc.):
https://support.saatchiart.com/hc/en-us/articles/205628027-What-qualifies-as-adult-content-?
Quote: Our curators decide which works qualify as adult content, however if there is a work you'd like to call to their attention, please contact us at [email protected].
You have the option of blocking adult content:
https://support.saatchiart.com/hc/en-us/articles/204628558-How-do-I-block-unblock-adult-content-
I should mention that none of my work has ever been classified as Adult Content at Saatchi, and I uploaded stuff that got me canned on Facebook by the nipple police.