Things don't have to *offend* me for me to think that my children (specifically, boys) are too young to view them.
Yes, that means an immature mind. But really, haven't you ever seen (or been, unless you're homosexual) a boy slobbering over the thought of "b00bies!!!!!"? How much less mature can a teenager act?
help.deviantart.com/220/ mentions the word "offensive" only in terms of "Strong Language" (conversation bubbles in drawings?) and "Ideologically Sensitive" material.
FWIW, I don't think you have to worry about this photo. It is sufficiently "artsy" and the "exposure" sufficiently minimal that tagging it Mature would only be a pro-active defensive action against the Overly Sensitive.