Honestly, I can agree with nipples being under NSFW. Why?
NSFW means Not Safe For Work
Not many workplaces have bosses who are okay with nipples being on their employees screens at work. If yours is, that's great, but your workplace isnt everyones workplace! That is important to keep in mind.
Wether or not nipples are sexual or not isn't really the question here - it's a question of it its safe for work or not. At least that's what I think.