Since a lot of tumblr users may not be old enough to remember, I’d like to remind fandom that the ability to write what you want about whatever characters you want was something that we fought for, not something that was ours by default. It was stigmatized, threatened with legal action, and mostly carried out in secret. You are living in a golden age of fandom, of AO3 and fanfiction.net and tumblr. Fanworks are actually entering mainstream awareness and becoming more culturally accepted. But let me give you a blast from the past, a look at what fandom looked like when the content owners decided what kinds of content they would and wouldn’t tolerate. From Fanlore, on the subject of Anne McCaffrey’s Dragonriders of Pern series:
From Southern Enclave #30 (Autumn 1991): “She [McCaffrey] has her guidelines (no sport dragons; no using her characters in our fiction; no men on golden dragons or women on bronze dragons), and a couple of Weyrs that violated her tenets were closed down because of that (and those Weyrs that allowed silver dragons were told to get rid of them— except for Ista 9)” 
In the FAQ list of the newsgroup alt.fan.pern is the following statement by Anne McCaffrey herself (dated October 8, 1992):
“The rules are that my characters may be referred to but not used. BUT there can be no adventure/stories set on Pern at all!!!!! That’s infringing on my copyright and can bear heavy penalties – particularly right now when there’s a film deal (yet another) which has bought and paid for the right to use the material – which, I fear, e-mail users have not. On CIS, I have asked people to limit Pern material to a discussion of their persona and dragons, fire-lizards, etc., in a diarist form. Fanzines have slightly more latitude as the zine is usually mailed only to members so that’s limited publication, and a due copyright notice is included. As there is no such protection on electronic mail, we authors have to be insistent on these safeguards. I know this can be confusing since Paramount and Star Trek are handled differently, but that’s the point: they are, and have been. Individual themes and characters of s-f/fantasy novels are not. And such indiscriminate usage of our characters, worlds, and concepts on a ‘public’ media like electronic mail constitute copyright infringement AND, which many fans disregard, is ACTIONABLE! Both the e-mail company AND the person. My publishers are most insistent on that point! So it’s to safeguard the interested e-mail user that I make these very strong, and perhaps unpalatable points.” from the Pern Encyclopedia
Initially, McCaffrey set out the following guidelines:
- Fanworks are strongly discouraged outside the approved “fan Weyr” clubs.
- McCaffrey’s characters are off-limits without prior approval, as main characters, minor characters, or even as background.
- McCaffrey’s primary setting, Benden Weyr, is also off-limits, as a story location, or even as a point of origin for original characters.
Eventually, however, she relaxed those restrictions, and, after 2004, only insisted on:
- Fanwork being non-commercial
- Trademark and copyright notices
- No pornographic sites “based on [her] literary works”. 
When content creators can decide what is and isn’t acceptable in fandom, it can be “no rape jokes” or “no noncon/weird kinks,” or “no porn whatsoever” or “no slash” or “no using my characters.” They are not required to be sex-positive, non-homophobic, or even reasonable. When one content creator gets that power, they will all want it. And they will not all use it in the same way, because our content is created by many different kinds of people. Would you trust Orson Scott Card to have the final say on what should be okay in the fandom for his works? How about Terry Goodkind? J.J Abrams, Steven Moffat?
Fandom as we know it cannot exist with restrictions on what kind of content is acceptable to the rightsholders. That’s never been how we rolled.