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I havent seen any, I am saying my friend's drawings have them and I wonder if they need to be marked as NSFW

That's controversial issue. Most likely yes, but... Can you throw me a link?

Its on my phone, its a photo of some graffiti she has on her house :/

Well, look. It there're viginal and penises - it's better to put the tag. About nipples...
We had this discussion before art-trail contest with nudity as a task. Opinions divided 50/50:)

which side do you stand on?

Well... I don't see anything bad in nudity as long as no one forces me to look at his own intimate parts of body:)
When it comes to drawing - I think it's completely OK:)

I see what you are saying, but maybe people don't want a thread with nipples, even if they are cartoon made, its tricky.