The reason this needed a disclaimer is because a LOT of erotica is pretty much derogatory crap (or worse rape stories) written by people who don't really understand the subject matter (ie: 50 shades) or don't understand what it is that /makes/ something hot, let alone what is good writing. If people just grab random erotica and read it under the impression it's going to help them, well it's probably not.
All of this is entirely subjective. While I've never felt disrespected or criticized by anything I've read I do agree that there is a lot of erotica out there that could be considered poorly written. That said I've seen some erotic series that I thought were terribly written gain a very rabid following. I feel erotica, more than any other genre, shows us just how diverse we are as a people. This is all thanks to the fact that we don't have publishers declining to publish stories because of their personal feelings on the matter. If someone wants to write and publish something...they can!
This has led to an amazingly diverse genre that has, literally, something for everyone. When you have a genre this diverse a lot of it, possibly most if it, won't be for everybody. Whether it be a style of writing or the specific subject matter it is going to resonate with some people and not with others. I have a peeve where the use of the word "rump" to describe someone's ass just turns me right off. Not even sure why, it just does. Because I don't like it I would not say all books that use the word rump should have a disclaimer on them, however.
As for your mention of "or worse rape stories) believe it or not rape is one of the biggest requests my wife and I get as writers of erotica. There is a large demand for rape fiction and it's not all from men, I'd say half of the requests we get come from women. Again, everyone is different in what they find sexy and erotic. Now I'm not saying that just because there is a demand for it you should write it, I'm just noting that there is a shocking (to me) amount of demand out there for very hardcore/violent sex.
If any kind of disclaimer was needed it might need to warn newbies to erotica that they might want figure out their own hard limits and learn to identify what stories may offend them and avoid them.
There's a /lot/ of erotica out there, a crazy amount. Everything from Dinosaur porn to Bigfoot to supernatural romance to office romance to every other kink you can imagine. Not all of these make for good reading or help write anything remotely good when it comes to TS.
If you want the best bang for your buck, so to speak, /TALK ABOUT IT FIRST/. Don't just drop sexy times on your partner and muddle around grasping at straws. Talk about what language is good, what limits and squicks might be had, even what person (When dealing with two individuals of same gender 'you' can go a long ways to differentiate who is doing what rather than she/her he/him/his or whatnot).
Communication is key. No two people are going to like absolutely everything the same to the same amount, and it takes some effort to get things to click as best they can for everyone's sexy times enjoyment.
This I agree with very much. As a player I play both male and female characters and I've had both sexes just sort of jump right into sexy goodness when I wasn't ready, prepared, or willing to do so myself. I don't think great in depth talks are necessary but certainly making your partner aware of your own limits (or lack there of) might be a very good thing. I think Shang's +kink system works very well as a passive way of setting up what you're willing to do as a player. Barring that I think liberal use of +finger notes is a good thing too. Pre-emptive communication as it were.