I was on the fantasy-side of the fence, but her back complaints seem out of place if nothing actually happened ... but ... can't say I'm certain either way.
This story's remarkably big on ambiguity in general if you look at it more closely, besides the possibility the sex was all in Sakura's head with or without (given the "mental intrusion" abilities implicit in the original lore) Noriko's psychic participation - mindfuck sensu stricto? :P
Eg. it seems to be implied Noriko's succubus nature is something of a secret or at least something she prefers not display openly, yet Sakura offhandedly dismisses it as "not mattering"; is this because such beings are no stranger a sight than foreign tourists in this setting, because she doesn't know who to tell without being laughed out as a lunatic ("who ya gonna call?") and Noriko doesn't seem to be up to anything actually objectionable anyway, or because she's had some kind of mental block implanted by way of demonic mindfuckery sensu lato to uphold the Masquerade?
Similarly much of the relationship between the two girls is left vague, beyond Sakura's obvious giant tsundere crush on Noriko and the latter's amused awareness thereof. There's hints that they're fairly regular fuckbuddies, and apparently Noriko views Sakura as her (current?) "favourite human" - but then this is contingent on how real exactly you assume the sex scene to be (and if it's entirely in Sakura's head then her raging homo lust is Brobdingnagian indeed...). And should it be taken to be real - physically or psychically - rather than, effectively, mental masturbation there's the question just how much you can take a literal demon whose whole gig is manipulating people sexually at her word...
Hell. If you really want to shake those Epileptic Trees you might even postulate there's nothing supernatural whatsoever going on and these two are just engaging in some kind of strange, sustained and rather chuuni form of roleplay... :P
(not that I'm entirely sure what a back that feels sluggish means. Is she talking about her head actually?)
Same. It is at the very least some quite peculiar wording.