Huh, the madmen actually did it. I was skeptical that this would ever actually exist. Probably years and years until they finish it, if they do, but they released it at least!
I have to say though..why are people so against NTR that they will skip a game because it has a bit? Is it because they are so attached to these fictional characters that it bugs them to see them with a different fictional character other than the one they control? I can understand for games like NTRPG and THP, but something like this? Serious honest to God question here, no sarcasm.
Since you ask politely I will try to give you a general idea as I understand it. Hopefully, the Dev's don't delete this for being 'off topic'. Also please note that this is a gross oversimplification and I am glossing over many debated points.
While the real answer to this question is no doubt as diverse as each individual person. It likely has to do with an individual's approach to sexual attraction, stories, and personal morals. Traits such as one's Demisexuality, orientation, and personal view of sex as a physical or emotional act. Combine with an individual's approach to a story. Some see themselves as the MC some as a 3rd party observer.
Combine again with personal morals, specifically related to sex. This isn't a right/wrong or good/bad morals. One may see their partner getting off with someone else as a way to make that partner happy. Others may see this as a betrayal or being left out. Why a person sees this one way or another is a complex combination of nature & nurture. Personal trama is also a big 'X factor in this.
Basically (and again I stress,
oversimplification) Take someone's rating in each of these areas, assign them a value, graph the result and you will have an accurate indicator of what arouses them sexually. The inverse is also true so it is also possible to take the opposite (invert the graph/values) and you will have a basic model of fetishes they like and dislike.
People saying I dislike 'X' or can there be 'Y' are just the results of these personal factors manifesting. It is also why some people see a tag like 'Futa' and don't care, whereas others lose their shit. Or some people say 'NTR' is exactly (insert their strict definition) and (even though they may be correct technically) don't understand why 'NTR-haters' are still pissed. Skippable/Not-skippable, Avoidable/Unavoidable, these are all values in that personal graph and have a value of importance to each individual.
Sadly, it also explains why debates around the value, inclusion/exclusion of 'X fetish' will never end. Even if everyone was 100% honest with zero trolling, everyone's 'perceived value' (a whole subject to itself) is different.
Anyway, I hope this gives you some insight into the psychology behind it.
If your truly interested in the subject I suggest reading; The Psychology of Human Sexuality by Justin Lehmiller. It's available on Amazon for a couple of bucks and is fairly easy to read without any prior knowledge of psychology. It's not an exhaustive answer (it is a very complex subject), but it does present many of the generally accepted principles of psychological factors.