Are you considering implementing a different system? Please, hear me out.
I'm 24 and I have a 10 year old brother, whom I'm trying to get into the world of 3D. So naturally, I opened Sketchfab (as a guest, not a user) to show him some cool mech/architecture designs and scrolling past exaggerated erotic sculptures left me feeling uncomfortable. It's not even a matter of "contaminating" his young mind, because since the dawn of time young boys found ways to get to adult content, but why can't it be displayed only to adult users? Why does it have to be so easily accessible? My problem is that my parents might see what my brother browses, and they'll think I granted him access to a pornographic site, instead of a place of education and inspiration.
And about your interpretation of artistic nudity - let's not fool ourselves, this place is a haven for neckbeards sculpting their erotic fantasies. I've had years of lectures on history of art, I've been to many galleries and I've seen examples of artistic nudities, there is some of it on Sketchfab, but maybe in a proportion of 1 to 10, so maybe we shouldn't treat the purely pornographic depictions as exceptions, but rather the other way around.
I really wish you would implement something to tone down the boobs for guests and young users on your website, because other than that, I think it 's great.