I think many people here do watch porn, including those who cry NTR at every possible opportunity, even if there is no NTR.
Since my argument was not directed at you, much less only at you, your counter argument isn't valid...
Anyway. I do get the idea that the devs wanted to make a somewhat realistic story. A realistic story is going to have other guys (not the MC) fucking potential LI's (assuming that IRL every girl is a potential LI) thus there is NTR everywhere IRL. That is my argument.
Of course the reality is that IRL only a few women out of the billions on this planet are potential LI's. The vast majority of the others you won't ever meet, and of those you do meet, a lot are either not interested in you or you are not interested in them for whatever reason, or they are already in a relationship. It would be really sad if a person would feel jealous every time he sees some random couple walking hand-in-hand or kissing in public for example.
That there is porn logic on other parts of the story detracts from the realism but that is only sideways relevant to this discussion.
As for non-NPC incest: I recommend checking out Fetish Locator. There is a brother-sister couple, the brother is the MC's best friend, who fucks his sister but is also into watching his sister fuck other men, including the MC. I would not have thought this was my thing (it still isn't), but the scenes are really hot. Since the sister is only a fuck buddy / fling for the MC and not a LI, this isn't NTR either. As an aside, the MC's own sister IS a potential LI in that game.