Hmm... One time I wrote a scene in a Game of Thrones RP starring a lord who was a pedophile, which was a rather divisive character to begin with. Then his intro post was him down in his dungeon of shame and having a young boy brought to him. Guy acted all nice to the kid while thinking things along the lines of "oh gods no this is so wrong... but I'm gonna do it anyway." The scene ended with the guy placing his hand on the boy's shoulder, with heavy implications of what came next. That one made a lot of people in the RP rather understandably uncomfortable.
Then there was a scene I wrote for a The Walking Dead themed game of Rabbit Doubt (it's very similar to the game Mafia, murder mystery type of thing). One of the killers was creepy as fuck and had a woman tied up in a chair and blindfolded, and he did some creepy shit with extremely sexual overtones like making her suck on his finger. The site had a PG-13 content rule so that's as far on the smut scale as I could go. The woman tried to convince him that he didn't need to kill her, that she'd suck more than just a finger and never tell anyone what had happened, but then he stabbed her (with a knife and more intercourse innuendo) to death.
But I think what probably takes the cake is some of the weird shit I very briefly wrote about in World of Warcraft roleplaying in order to humor some of my more.. interesting roleplay partners. I did a bunch of sexually explicit roleplaying in those days because young adult male hormones plus women (some assumed, some more or less confirmed by picture and/or voice) asking to do sexual things via text equals greatly lowered inhibitions. I'm not gonna go into great detail here, but I'll give enough to paint a fairly indicative picture. In WoW there's a race known as the Silithid, which are large and not very humanoid insects. Ovipositioning is the term used to refer to something laying eggs inside another living thing, often in a fetish context. Vore is the term used to describe the concept of someone being eaten whole, usually in a fetish context. So, with that in mind, I once wrote a couple male humans.. interacting with a female Silithid in an ovipositioning/vore scenario. It was hilarious and awful and one of the least arousing things ever, and that was just one thing among dozens of weird things I wrote/roleplayed over my years doing WoW RP.