if butts go free i think all nips should go free as well... like the argument "there's a stigma, i can't have this on my screen what if my boss or family sees it!!" i'm p sure your boss would not wanna see a naked ass on your screen anymore than a lady's nips. both have "stigma" to it and are considered inappropriate.
personally i'm a fan of free the nips & the butts like... they aren't that offensive. while younger ppl may be on this site i dont think the main audience is 12 year olds. if a reference picture of a woman simply standing there, existing with nipples is upsetting to you then well... that is a problem with you and the way you view women. ppl have some responsibility to control the content they consume. like obvs if they are being displayed in a sexual mannered they should be marked for sexual content, but if it's just a person in a ref or something like a mermaid swimming freely through the sea... there's nothing explicit about that.
allowing all of this to just be viewable would also eliminate the whole linking offsite thing
like i get you can't please everyone, in the end whatever is decided, there would be an unhappy party. i think it's best to go with the solution that is also kind of practical as well? so that it's not too complicated to implement or enforce.
the mild nudity filter can stay for those who WANT to use it, but maybe just make it a suggestion for sensitivity to your followers and not a requirement. if ppl have followers that ask them to tag for nips, the artists has the choice to do so; but if their followers dont care and the artist don't care, they don't have to