I don't see how that's a character flaw. You're basically getting mad at someone for something they didn't do. So what if there's an alternate version of them that cheats on you? By your logic, should we start making games where the love interests hate and avoid you by default because there exist routes where you could choose to hurt them or romance someone else? You're in control of what happens to your love story or whatever. Who gives a shit about the branching paths you won't take? If you don't let it happen, it just doesn't happen. You are in control.
I think you're reading too much into it. Of course it'll be written somewhat convincingly for people who are into NTR. Because why the fuck would I consume NTR that's intentionally unconvincing? No offense, but it's kinda silly to say that a character with optional NTR content's "true nature" is a "cheating whore". These characters have no autonomy. They cannot act outside their predetermined set of actions. They cannot leave you of their own accord. They're only a "cheating whore" if you want them to be. You're letting your real life anxieties bleed into a game.
Cheating on their partner the moment some rando sticks a dick in them isn't a character flaw? Lol. Maybe you skipped over my example of Seed of the Dead. The characters are very much pieces of shit in a way in the NTR content that even devalues them in the non NTR content as somebody not into NTR. That shit is super common. And no, the MC is not in control. The player may have control to avoid the content, the MC does not in those situations. You turn NTR on and some dudes go and fuck the MCs "love interests" behind their back. And they fall madly in love with randos cocks while badmouthing the MC. NTR toggles are usually horse blinders blocking how terrible the characters are. If they were written in a way so it was clear the NTR was an exception, that would be different, but it rarely is written that way.
You're not even trying to see things from other people's perspective. "You're reading too much into it", while attacking people for their preferences in H games, lol. Try not to so blatantly contradict yourself and make it apparent you're just trying to shit on people who don't like NTR. Starting off by throwing out insults to anybody not fond of NTR, and then a personal insult at me for sharing my perspective. That the kind of person you wanna be? "No, you're not supposed to enjoy H games like that, you're supposed to enjoy H games like I do". If people want to actually care about and get attached to characters, that's on them. Some of us aren't interested in characters that have shitty traits. Stop trying to control people and let people enjoy their fictional porn games how they want to.
Just for an extra little example: Imagine if you could read minds, and somebody close to you was constantly thinking nasty things about you, or thinking about torturing puppies, or raping children, or whatever would greatly upset you. Even if they never acted on any of those things or said anything bad about you out loud, can you honestly say that wouldn't hurt your opinion of them? Logically you can understand most people have intrusive thoughts now and then and it's pretty normal to a certain extent, but to actually hear it is different, right? They may not have acted on it, but you still know.
In the "Optional NTR" route, it's not even just thoughts, you know those thoughts have been acted on before just because some rando stuck a dick in them lol. The potential is always there, somebody raping them or propositioning them in a certain way is going to make that "cheating whore" version of a character "reality". It's out of the MCs hands, it's up to all the other fictional characters in the world not making advances on the girl for her to not show that side. When the girl herself doesn't drastically change before the sex with rando stuff, sure seems to me like she's basically a cheating time bomb waiting to explode, lol. I don't think I've ever seen optional NTR in a game not show that the character is kind of a piece of shit even without the NTR. Maybe if she's drugged and/or raped/tortured to shit and ends up with stockholm syndrome, I couldn't blame the character in that case. But you and I both know the usual NTR fare leans towards the girls falling to any rando cock with little to no setup, because the NTR gods demand it : p