I'm glad to see that I'm not the only one who feels this way when playing NTR games.
When I play a game, I like to put myself in the protagonists' shoes and tend to take things to heart, even to the point of hating certain characters and killing them (in-game, of course).
When I managed to get rid of the Dragons who were the source of the world's evil in Dark Hero Party, you can't imagine how good I felt, but after defeating them, I felt a great emptiness and a kind of disappointment. Dark Hero Party pushes the player very far into this kind of feeling, and I would have liked a positive ending between the MC and his childhood friend, but we're straying from the subject here.
I never got the courage to play that game for the reason you mentioned. Or maybe I could comfort myself thinking it's my wisdom, protecting me from things that it knows will hurt me.
Nevertheless, I would have liked to see a game where the character loves the MC (but doesn't sleep with him) and when he discovers the deception, he wants to leave her for good and she does everything to dissuade him. That would be really new and unprecedented in a game of this kind.
I would have liked the player to have more relationships with the other girls in the group (whose names I've forgotten).
He's content to rape the other girls and even the teacher Melissa (who's a lesbian) instead of accepting the advances of the other two girls in the group who seem to have developed feelings for him because of the various loops. That's what's wrong with him.
With magic, the MC could have increased the size of his contraption or found a cure for his premature ejaculations.
He could also have developed a feeling for the Demoness with whom he had sex too.
I'd have liked to see whether Amelia would abandon the hero if he left, because despite her sex addiction, she seemed to want to fight and continue working on missions and protecting the MC. If he was frustrated after what she'd done with Leon, in front of him, and threatened to leave, I wonder what she'd do, but we'll never know. Maybe she'd become Leon's slave and abandon the MC.
Yes, those ideas are all valid, even I had plenty on how to make the game more interesting as I was playing. I think that and also the fact that I'm actually fairly numb towards this game stems from the fact that it's just simply not that good. Even the simple suggestion that MC ended up with Hannah instead could turn the whole game around and give to the player the feeling of fulfillment.
Being a creator at heart, I'd like to be able to create games that are out of the ordinary, because I have a feeling of déjà-vu.
I'm more like the annoying guy that tries to find plot holes in everything

, simply because of my obsession with realism - and yes, even in fiction, "custom rules" doesn't mean "no rules". I've ruined many stories from my childhood this way after thinking about them for long enough.
I don't know about you, but I could never be corrupted by Sex. I'm not saying I don't like making love, but after the act, the effect fades and I suppose it's the same for girls... of course, I've known nymphos who loved sex, but they weren't thinking about it 24 hours a day, so it's obvious that all this so-called corruption is only fictitious and unrealistic in real life.
Some NTRs don't emphasize these inner experiences with those she cheats with others. Whatever the feeling linked to sex, it's ephemeral although it can be redeveloped at any time, and in my opinion it's not as important as all the years of happiness Amelia and the MC have spent together.
As a person who has a strong sense of justice (or at least I'd like to think so), I was at one point frustrated with the idea of infidelity, to a point when I decided to research why and how it happens. The studies I've read gave me many interesting insights, and I would strongly disagree that corruption itself is fiction (corruption through sex still is), to a point that might surprise you.
For example:
Amelia has spent her whole life with the MC, it's inconceivable to imagine her leaving him for Leon or anyone else when she's known them for less time than he has.
This in particular could easily be realistic. Of course, statistically such relationships are destined to fail - the chance of success is about 9% or so (hard to say, but it's the most optimistic number I remember), but people who cheat will, to say it briefly, experience "blindness" (can happen for a variety of reasons), and will realize their mistake later - often not that much later.
Perhaps a bit more realistic take on NTR would entice some people, but alas, I was not gifted with much talent for writing, so I'll just have to wait for someone who was.