There's a limit to how much content can be in the game? Why not have 500 pairings?
But if you look at it from another perspective, Carwyn has more variety. Gwenivier only has Carwyn, or three flavors of NTR. Carwyn has Gwenivier, Vivian, and being used by the rest of the other guys in the group. So he has a five person to four person advantage over Gwenivier.