Great post, and I'd like to add to your argument about people being irrationally afraid of nipples, but I agree with it completely and can't think of any substantive.
With regards to the NSFW tag, there is one pragmatic side to consder. I do appreciate it as a browser when there's nudity (artistic included), as I often curate in public settings and would rather just avoid getting dirty looks/harangued by people who mistakenly think I'm looking at porn in public if a nipple shows up.