I have to say that, as a reader, I never expect sex in a story. In fact, I usually don't even enjoy the sex scenes...well, that's actually a total lie, but the best parts, in my opinion, are all those things that lead up to the sex -sexual tension in a story is what will keep me hooked forever. In one of my favorite stories on AFF, the characters haven't even come close to getting together (I don't even think they really like each other yet), but the author puts them in these situations that just make me want to scream, they're so tense. And, though she hasn't updated the story in awhile, I constantly go back and re-read it, overcome with jealousy at how well she writes those scenes.
If the rating on your story just says ADULT with no +'s, I don't know why your readers think it should or would have citrus in it. However, posting in the Inu fandom is really kind of a pain -as others are, too, I'm sure, but I have only experienced the wrath of the Inu fanbrats. Most of the readers there seem to always want to see those characters getting naked and figure good writing can go to hell (and then they bitch that the characters are OOC...you just can't win). I've read some stories on there that were terrible and then I've looked at the reviews and was astounding to see things like, "This is the best story I've ever read!!!" You're kidding me, right? But people love it because there was sex in it (not even well written sex! No tension or anything! Just off with the clothes and into the bed. Ugh).
I say you just keep doing what you're doing. If it was my story, I might even add an author's note replying to reviewer's concerns about the lack of lemons. The politeness of the A/N would probably depend on how frustrated I was while writing it, but I would at least try to be civil I also suggest that you keep the story up; you never know when that one reader will come along and totally appreciate what you have written. Lack of reviews sucks, and bad reviews are even worse, but, for me, one good one can keep me motivated for months...but that may just be me