If it's primarily one person, it might be worth separating it into an extra chapter of its own. Depending on how large the whole fic is, and how conveniently the sex scene placement is, anyway. I guess it'd be awkward if it were three paragraphs in the centre of a very short fic.

Oh! It just occurred to me: can you code the text white, so people can just highlight if they want to read it? (Maybe with the sex scene at the end of the chapter, if it's a multichapter fic, so those who don't highlight can just scroll and hit 'next chapter' to continue?) Obviously won't work if people are using fancy AO3 skins, but I don't know how common that is.