Because nobody sane wants to watch a love interest with another dick.
Well, no kink shaming. And questioning the sanity of people liking stuff you do not like. That makes you very quickly a bigot. Like people hating on gay stuff and claiming gays are sick in the mind.
Also, it is not your dick in the first place, so you already see your waifu with another dick. Especially in kinetics there should be more distance, with the little or no control you have over the story.
There should be another word for what you say. Like dicksclusivity or a simple dick-counter. Because if I understand correctly, you would not even want to see the prot spitroasting a girl. So if games would simply state dickslusivity or say, there are multiple dicks in that game, no need to investigate further, if it is real ntr or something else. (Oh, and for real ntr, you need not even see the other dick. Also, you could be cucked by the lesbian lover of your wife)