Indeed, there's a bit of sadism in it.I always find it strange of how much NTR shows "loser" dude crying. I think NTR enjoyers mostly getting of this shit.
Netori is more about corrupting girl and her fall. Husbands/boyfriend are only mentioned in text and are just afterthought.
I never liked to read through some "loser's" lamentations but I guess NTR enjoyers are full of envy and are getting off some dude's misery.
I'm not necessarily an NTR fanatic but I can still see some elements of appeal to it.
In the end, it's just one of many fetishes/kinks that not everyone will understand. And that's okay.
So when it comes to implementing such fetishes in games, the best way to do it is making it an option. It being an option is a win/win for everyone.
Ofc this implies a lot of work for the dev, but if the dev can afford to put that work in, it can only serve to benefit the game by further enriching the gameplay and expanding the player base.