It is a hard juxtaposition to balance, when considering dual-MC games, and there are at least a few who get it right, thankfully.
Having an MMC that keeps the player satisfied with his characteristics, be they meek or dominant, depending on the situation, whilst still not making him some simp/wimp who may as well not even figure in the story, he has such little influence on outcomes.
Having an FMC who embodies the desirable traits for the story, while still engaging with the MMC in an appropriate fashion, is also hard, because the tendency is too often just to turn her into some cock-hungry cheater, instead of a reliable, honest and cooperative partner in adventures.
Here, in this thread, we ask a lot of devs really, but hopefully there are a few who may come and read these comments and become inspired to create more inclusive stories amongst their "deviate" ideas, for us to enjoy as much as the ubiquitous and vociferous NTR-cheating-slut crowd seem to get, seemingly without even asking....
In the meantime, with the dearth of actually great sharing/swinging games, its just as well our combined tastes are varied enough to able to go off and play other genres as well, right? Or it could get dire indeed in here, haha...
(no offense to this great list Johnny)