Personally, I'd find it interesting. Anyone that criticizes a game for doing that AFTER you gave them the clear choice to play the game this way can fuck right off imo.
It's with any fetish or kink. Even the ones not allowed on here because we are talking about 3D Assets and pixels, not real people.
If you've given fair warning and potentially even the option to disable something, and the player still complains then I can just shake my head and facepalm.
To be clear: I differentiate between discussing and complaining. Suggesting to include something with some (subjectively) valid reasons towards a dev or saying something shouldn't be in the game because x, y or z (it wouldn't or would fit the tone for example) is not complaining.
Back to topic:
Depending though on how strongly an open relationship for the girls would be implemented it could create a unfavorable cost-benefit equation. A one-night-stand not shown is easily written. But for anything more serious it would necessitate a whole new character for something potentially not a lot of people would find interesting.
One upside though would be to set the game apart from the rest. After I played Radiant I had to play Thirty Days again so I could set them mentally apart. No joke, I literally confused them because they (currently) are so similar.