It already is, and they already have/do!
Ik, but they are going to moan more up to the point where the dev would abandoned the game just like many other did before..
Because NTR sucks and only uncultured swine like that cuckery.
NTR for me in VN is part of realism, trying to work around it and make the right choices to not fuck up and lose my women/s just feels rewarding and it works like real life you know?
Or do you actually believe that you are the only person with a dick on this world/universe and no girl ever had sex with anyone before waiting for their prince charming (you) with his huge "DICK" to take their "virginity"?? YEAH! this sound very realistic and it make total sense just like IRL...
Also, I don't have to put myself in every goddamn male character shoes when I play a fuckin game... it's just like watching a movie or a series where the females cheat on the main protagonist/s "OHH NOO I AM BEING CUCKED, NTR UNAVOIDLBLE, FUCK ADD NTR TAG" wait a minute... I don't give a fuck about him! it is not me who got cheated on!
What about porn movies? if I watched a gang bang video (or any video for that matter) for example am I being cucked too??
Real life sucks and I hate to break it to you, but your brain is the only thing being cucked here by it's own cells buddy...
Edit: I forgot to mention the games with prostitutions tags with avoidable or no ntr in them, can you imagine PROSTITUTES who only have sex with the main character....

Examples:
BangCity,
Depraved Awakening and
Summertime Saga