I'm honestly surprised this is even an issue, as published fiction comes with warnings all the time -- genre labels, cover art, back blurbs, reviews, all sorts of miscellaneous media that tells you what to expect. There aren't specific content labels, but judging by the number of people I've heard complaining about being blindsided by graphic rape scenes in fantasy novels, maybe there SHOULD be (Mercedes Lackey, I am looking at YOU).

Fanfiction doesn't have covers, blurbs, or extensive summaries/reviews. Warnings are, quite frankly, easier than generating all that miscellaneous stuff.