CSAM, commonly known as “child pornography”, is any media recording a real person who is too young to consent to sex with an adult, in any situation where there the primary reason for the recording to exist is to sexually titillate an adult who is attracted to the child.
Controversially, we also extend this to “loli” art — short for “Lolita”, after the titular character in Vladimir Nabokov's novel from the first-person perspective of a pedophile with narcissistic tendencies — and other visual and aural depictions of underage characters in sexual situations or situations that primarily exist to titillate an adult audience. “Loli” is legal in the United States, but the Fediverse is world-wide, and this material is extremely illegal in many countries. Many server admins do not consent to have this material mirrored to their servers, and “loli” fans have historically been bad at compartmentalizing their content.
This policy does not extend to the case of written fiction. Fiction, especially fanfiction, can serve as a useful outlet for victims of CSA to work through their feelings and process what was done to them. This includes sexually explicit writing about underage characters. However, such content must be clearly tagged, and should ideally be hosted off-site, e.g. on AO3, and not as part of the Fediverse itself.
We do not currently tag bestiality with real animals, or depictions of bestiality, but this is primarily because such content is uncommon and — to be quite honest — people who are into bestiality are better at compartmentalizing it rather than forcing other people to look at it. We will create a similar policy if the need arises.
For the moment, we do not generally prohibit any “furry” content, i.e. content depicting fictional animal-like characters with full human-level sapience, although any position in either direction on this matter will be inherently controversial. This currently extends to not prohibiting the depiction of characters that would be underage by human standards, but we may need to revisit that standpoint if such content escapes containment and reaches the broader network. Again, consent. Use CWs and create alt accounts on instances dedicated to that kind of content if it's escaping containment, or we will have to block you.