I think the problem is definitely that, what with site being Adult Fan Fiction, partnered to just regular Fan Fiction, readers are probably going to assume that anyone posting is posting something that has adult content all over it. Which to be fair, like someone already said, doesn't have to be sex, but I think thats what people expect.
I admit, I've posted a review once before that asked for two characters to get it on, but only in the sense of 'the tension you've built is fantastic, please break it soon!'. I certainly didn't mean it to come across as 'I'm only interested in the smut in this story' comment - I can imagine a lot of people assume smut will turn up eventually and if a lot of chapters go by feel inclined to remark on it.
A lot of reviews are probably innocently made. Unless you're insulting someone you probably don't stop to think too hard about how a positive comment could actually be annoying after all, if you're saying you like things and you want more, and more of a specific thing, how could that be bad? But I'm glad I read this topic, because now I will definitely be more careful when posting all reviews, not just negative ones!