For many years, my very favorite erotic stories have been ones where a man and woman are in what has been a faithful relationship to one another, but they're both horndogs, and there's some sort of catalyst event that leads to one or both of them having sex with someone else and it opens the seal, and they both then begin to explore other options while not leaving their partner, and it grows from there.
It might be something as simple as meeting their neighbors who are swingers, and being introduced to that lifestyle gradually, and having the MC watch his wife as she gets fucked by other men while he at the same time is getting his cock sucked and fucking hot women.
It might be something more elaborate (since this is all sex fantasy) where one or the other goes away to a work conference or event that involves casual and/or required nudity, and they end up having a bunch of sex, confess to their partner, and it opens up the relationship from there.
Or it could even be something wild that involves sci fi and reasons that everything could get started that way. The kind of things that are common in harem games, but it always makes me shake my head when only one man in the world is having sex. I want lots of people to be in on the action, of both sexes.