Sexual content generally goes in the Erotica sub forum as it is age restricted, aside from that, I guess it depends on how adult you are talking.
Sexual content, graphic violence, perversion, obscenity, and so on. Anything over eighteen I suppose.
I'm pretty sure there isn't a dedicated area but perhaps you could make a post directing over 18s to the Erotica sub forum. I'm sure the mods will have a preferred way to go about it, but it's all I can suggest for now. The site as a whole tends not to censor any more than it has to.
Erotica room is the most suitable place for posts with specifically sexual content that might not be all that suitable for underage eyes, say, in the Research room, but mostly we rely on the members' ability to trust their own judgment. If you're unsure, you can always ask via PM whether whatever you're planning to post is ok. We aren't terribly stringent in this regard, and you can see that e.g. in the research area there are also questions and topics about violence, torture, etc. If you post something explicit to the Writing Workshop, it's best to mention it in the title (e.g. Graphic violence, language, etc.). You need to meet a specific criteria before you can post your own writing for critique. Please check out our New Member Guide and Forum Rules for more information.