You know the best part of having a waifu who sleeps around? Threesomes! And introductions to other waifus
The way I see it, except in Anno and Kaedes case, the girls in game are more often the subject of an open relationship than polyamory.
Kaede and Anno clearly has a thing going with each other, and Kaede totally has a thing for Cass too. Anno don´t show t all that often, but her bedroom scene indicate a bit more intimacy between her and Steele.
So those two for sure are polyamorous. Knot a bad thing imho.
Now Syri on the other we only know fucks people other than Steele, but we´ve never gotten much hint of her being romantically interested in anyone besides the PC. (I´m not sure if the incest boner counts or not) So her relationship seems more like an open one.
It´s the same thing with Erra when Hugs´ xpac gets out. Coincidence? I think NOT!
Reaha surprisingly, being the one who´s probably slept around the most, is one of the most monogamous waifus in game.
I don´t think she´d mind an open relationship at all, but if that meant Steele would feel uncomfortable with her she´d rather only be with the PC. At this point, after curing her, it´s clear that she loves him/her and would gladly let the PC keep her all to themself. Which is pretty hhhnnnggggg!!!
If you want a monogamous relationship, Reaha is one of the go to waifus.
(I wouldn´t mind more threesomes with her though)
In the future I´m hoping for more waifu threesomes.
Waifus can love other waifus!