Okay this is amazing. I'm not sure why the harem thing is putting people off. As excellent as the dynamic of these two is, adding more into the mix could still be fun.
Actually, I've see quite a few comments on this site across various works, complaining when things look like they might go the poly route, and I don't really get why.
I'm not going to say it's most but a lot of yuri fans tend to read yuri for the intimate relationships between women generally and harems of the het, yaoi and yuri variety have a very high tendency to be badly written slop essentially where even if the girls individually are written well each relationship with the main character tends to be lacking in depth or true intimacy due to the nature of focusing on so many girls and relationships at once when most authors can barely write one.
Adding to this harem isn't really truly "poly" because it's exclusively multiple characters fawning over the main character with little to no inter harem chemistry to speak of because they must all revolve around the MC which frankly I dislike heavily.
Also this isn't related entirely to what I just posted but you have to remember that most of human society is monogamous and unless you were like a king with multiple wives (which is a whole different bag of worms and is much closer to men owning women as basically sexual or reproductive slaves) generally it wasn't seen as the norm so it's a niche within an already existing niche (WLW content) resulting in a lot of people not being down whenever something seems like it's going to go that route alongside the already existing issues mentioned above.
For my part I just want to see two girls of any size, race, monster/android/human, age etc in love and I don't really care for stuff that isn't that.