You sure you got the Sweet Ending? On it Rye doesn't try to bound the Protagonist.
Probably not then, I tried reloading and getting different endings by making some changes but after I got like 5-6 different endings from Rye route I kinda stopped trying and went for another futa route instead.
I think you are mixing some head canon there.
In fact the game is going the route of showing futaxfuta and futaxwoman are not a small minority. Plus it was never stated most futas aren't interested in relationships. The bounding is treated pretty much as a state policy and a divine right in the Empire.
Again this might be me having missed stuff, but I did not meet a lot of woman for that matter or couples of futaxfuta or futaxwoman. I mean from the top of my head (I did not see all content in the game) I only remember seeing 4 women, SaraxSuni which was a futaxwoman couple, the gym cycle woman, the clubs theatre woman and Wallis sister. Of these only SaraxSuni was a couple, the gym women complained about how hard it was for women to get dicked unless they were an incubator, the theatre woman didn't have a partner far as I could tell and the reason she got fucked seemed more because after the show of manxwoman the futas wanted to show their alpha side by fucking the woman to show what a real fuck was about.
On futaxfuta couples the only one I can think of that was seemingly in a real relationship instead of just having sex with each other was the police futa. But the view I got was still that even if there are relationships the futas mostly go for an open relationship or more of a steady sex partner then anything close to gettin married or being boyfriend/girlfriend. But again I believe I played maybe 80-90% of the game as it is in this version before I decided to move on so I might have missed quite a bit. Mostly I didn't get all of the endings since it seemed to be quite the chore to save scum without a walkthrough to get them all.
So again all that I've written is based on what I saw/heard in the game so if I've missed stuff that made it clear to be otherwise then my bad.