I've been seeing a lot of these types of doujinshi crop up over the years (particularly centered around NanoFate)
A tissue thin plot of maybe 2 pages of why they're on the couch/bed/bath followed by mostly samey sex. I feel like I can splice one or two of the NSFW NanoFate doujinshi pages that Nanashiki has previously made into this iteration and barely anyone would notice the changes. If feels like we're going through the same motions Groundhog Day-style
I can think of a few reasons why that would be the case with NanoFate. The thing is, while the anime never explicitly describes them as lovers, it does I believe have them in an amazingly close rock-solid relationship, cohabiting, and raising a daughter together. So there isn't a ton of room for plot there--the doujin world basically considers them wife and wife because, well, they pretty much are.
I also think that, how to put it, nobody wants to inflict plot on them because everybody loves them. Plot involves conflict, difficulties, drama, tension. Nobody wants their beloved NanoFate to be having that kind of thing happen; everyone finds their deep, stable true love very comforting and wants to support it.
So stories about them tend to be heartwarming slice-of-life vanilla, whether with sex or without. You don't read (or write) them for excitement or suspense, but to feel that relaxing "Ah, NanoFate are so sweet together, at least something is right with the world" feeling.