If a question is overtly offensive, it should be flagged as such.
Gratuitous NSFW pictures, links to porn, unnecessarily graphic language ... all of these are at best things to be edited out of a question immediately, and at worst might be forms of trolling. These are the kinds of thing which really need to be off our site. I know nobody will disagree with this; I mention it only to highlight the contrast with the kind of content you're asking about.
Mature discussions of NSFW content are fine.
We have hundreds of questions which are about sex in one way or another. As pointed out in previous meta posts on this topic, things like Kirk's sex life in Star Trek, Jabba's intentions towards Leia in Star Wars, or the too-many-to-count rape scenes in Game of Thrones/A Song of Ice and Fire are all perfectly on-topic SFF questions and can be discussed in a mature and reasonable manner. Often sexual things are an integral part of a sci-fi or fantasy work, and as such are fair game for questions and answers on this site (provided they're not being discussed offensively, of course).
So, coming back to your question ... story-ID is acceptable, and (reasonable) sexual questions are acceptable. Why shouldn't story-ID questions about sexual content be acceptable?
(I could maybe get behind the idea of using a tag for such questions, so that people can put it on ignore if they don't want to see any posts about erotica, but that's a discussion for another meta.)
Just to address the specific counterpoints from your answer
There are many places on the internet that one can find this sort of content. There is no reason SFF needs to be one of them.
This isn't an argument in favour of disallowing anything. There are many place on the internet that one can find Q&A about sci-fi and fantasy, but that doesn't mean we should shut the whole site down.
Erotic literature is several levels removed from the universe in question. First it is a fanfic and not canon in any sense of the word, and second, it is not intended to expand, continue, or expound on the universe.
But your question isn't about fanfic - it's about erotica in general, not all of which is fanfiction for a different canon. Again, consider A Song of Ice and Fire.
For various legal and SEO reasons, we are not allowed to post or link any sexually explicit or pornographic material. Any properly sourced answer to a story ID question looking for erotica would be pushing the boundary. And a question that goes into any sort of detail could be seen as explicit content.
Re. sexually explicit or pornographic material, see my first paragraph above. If people are clearly posting in bad faith, then they can be dealt with accordingly, but most of the community is able to discuss erotica sensibly. Re. detail in questions or answers on the topic - again, let's trust our users to be Nice and reasonable. Offensive content can be flagged and removed. It would be kind of disrespectful to ban discussion about sex on the assumption that the community is unable to handle it maturely.