Because they're writing for an audience? If a writer has a choice between two projects, and one of those projects is going to get a very vocal subset of people VERY VERY ANGRY, and the other isn't, which do you think they're going to choose? A character thus tarred with the cuckoldry brush is going to be an immediate turnoff for a significant segment of the player base, and thus less valuable. That's why writers (generally) have the champion be the one to initiate any threesomes or such, and why characters like Cait or Brint don't generally prompt threesomes for the champion to partake in.
But Cait already has a vocal audience of people that dislike her. Like, do you think that people that call her NTRCat are going to change their mind whatever they do to her short of making her player centrist? And if they did not want to deal with that, they would have not made characters as they are. Cait would be your girl, Brint would never exist to begin with. And look at half of Bubble cast which can at best be described as divisive. This game can appeal to many different people, which does in fact bring in many different points of views into this hot pot of a game. Which I do find amusing personally.
Still, I do not see your point in people accusing this game of making them cucks more then it already does. It constantly, and I mean constantly reminds you how much Cait fucks and shows you. Recent adventurous trio, her fucking your mother in law on your wedding and her constantly saying how many people she is going to fuck. Like, adding what I am suggesting does not seem like such a big step up to what is already present.
Same with Brint. Rune content, your companions talking about him and him fucking Sugo in front of you if what people told me was correct. How is that any less worth of complaining in eyes of the people that hate when others are fucking instead of them?
Only difference between him and Cait is that he is less active in scenes not focusing on him and that he can be replaced by someone who is more up to peoples taste. If Wsan only had him as a companion, then I can see why that would be a problem. But since he is not, who gives a hoot!
Now, the stuff about having to many promps, yeah I can see that. But there should be a way to regulate it. Some example as locking it behind if you have fucked Cait/Brint or just limiting number of promps that can show at the time. Just something to think about.