I'll take a swing at an answer if I may.
Think of it like this: NTR, sharing, and swinging utterly alter the characters involved and the story itself in fundamental ways. It changes who plays it, what audience you attract, the baseline personalities of the characters in the story, and the tone of the work. It's similar to scat, gay, necrophilia, bestiality, rape, etc. Even if it's optional, it often does irreversible harm unless fully removed, and unless that is the tone you want as the majority tone of course.
To further illustrate why it matters and how it changes the character involved even when optional:
Imagine the MC walks into a room and sees a love interest (or any female character) sitting on the couch with a dog. She says, "Hey, do you want to watch me bang this dog? I love fucking dogs but haven't done it in a while since I didn't have one, but I went and adopted this dog so I can do it... if you want to see it." Even if I can say no after that, it changes that character forever. She wanted to do that and has done it before. I'd lose all interest in her, and I'd bet a lot of other people would too. Now, what if she says it like this, "Do you want to watch me bang a dog? I've never done it before, but I saw a video (or something) and it looked fun. What do you think?" Same thing, I wouldn't want to be with someone who wanted to fuck a dog, even as a curiosity. It would alter my view of her fundamentally.
Another scenario is that the MC walks into the room, sees the girl and dog, and thinks, "Do I want to see her fuck that dog?" Again, even if I can say no, it still creates a disconnect between me and the MC because I wouldn't even think of that. The fact that he did means that he wants to see it, but I just didn't allow it. I'll know that there are images where he and the girl did it and I just "avoided" them. I now have no interest in playing a game as a guy who devalues women so much and would want to watch his lady love fuck a dog even if, in this moment, I made him not go for it.
A third possibility is that another character, say a friend or enemy of the MC, walks into the room and says, "Have you considered if she should fuck that dog?" Then I wouldn't ever think, "Hmm, should I?" No, I would kick that person out and end any relationship with that person because if the MC entertains the idea, then we are back to it might as well have been his idea, which ruins him for me. If I were in the MC's shoes and someone tried to make a move on my girl, I wouldn't even consider it. That the other guy would be lucky if he went home with all his teeth. So if the MC does anything less than that, he isn't the kind of guy I want to see a story of, especially if I can name him and make other choices for him and he had been, up to that point, functioning as my avatar.
This isn't to shame anyone who likes these fetishes (bestiality, NTR, or any other). I am simply pointing out that their inclusion has an effect (often "negative" since they play on negative emotions) even if it is "optional." Similarly, think of scat or necrophilia... those are highly polarizing fetishes, and knowing that a girl even wanted to do those things would turn me off of the girl and likely the game. And most players are the same.
The story and the character in it don't have a will of their own. The story will not "demand" anything and nothing will "fit" or not unless you want it to. You create it and every aspect of it. Period. If you don't like bestiality, then that will never show up, even if a character has a pet dog already. Even if some "fans" may keep saying you should add it to attract more players and just make it optional, because if you don't want that content, then it wouldn't even enter your mind to have that as a plot line. If you rule out incest, pregnancy, necrophilia, or scat, those will never show up. And if you like them and want them, then they will be part of the story and you will make them fit because you are in control. Even some thing like anal, which is often treated as a "vanilla" fetish, isn't actually vanilla and can have a detrimental effect on a character if not handled well. If I'm not into anal, for example, and there is a girl who loves it and it's "her favorite type of sex," then even if I can turn it down it has fundamentally altered her character. She has done it before, she enjoys it and wants it, I'm just denying her what she wants and I also have to be reminded that she's done it with other guys before. Now a girl who is curious about it and maybe used a toy or only stuck a finger up her butt, sure, maybe not as bad, but could still be a massive turn off for some people. And think about the people who, if a game has a trap, don't even want the trap to show up or want them to just be rendered as a girl. Because even the hint of gay sex or that the Mc could be attracted to that guy changes the characters fundamentally even if they can turn it down.
And all this doesn't even go into how "optional" means it still happens. There are still gaps in the story where a girl went "out last night," and on the non-NTR route, nothing happened, but on the NTR route, she got fucked by other guys and we saw it. I have never once seen "optional" NTR implemented in a way that didn't have bleed-over and story gaps where that NTR was taking place. Worse, in many stories, the dialog often still holds artifacts of NTR. The worst are the many times where broken code has hit people on a non-NTR route with NTR scenes. Nor am I going to spend a lot of time on how each moment you spend rendering NTR scenes is a moment you aren't rendering the scenes that are actually the main draw and point of the story (unless you are making a full NTR game, of course). I also won't go into too much depth about how optional NTR pleases no one. People who hate it will leave the moment it is added, and people who like it will be upset if it isn't a focus and is only with one character or a one-off scene. (EDIT: And this is partly why you get complaints even when it is labeled as being there, because people see "optional," and assume you mean it is actually possible to play the game as if the NTR doesn't exist but NTR by it's nature changes so much about the characters and story that they play and still get hit with the fact that the NTR is there, they are just not seeing it.)
I also won't spend too much time talking about how some NTR fans will actively bribe, threaten, dox, skew polls, and more to screw over non-NTR games because for some of them, the idea of screwing up a vanilla game and fucking over vanilla players is their real fetish that they get off on, and the NTR is just the tool they use to do that.
Finally, I'll end with this: Sexuality matters to people, and people have (and are allowed to have) standards for what they invest both time and money into (and time is money) engaging with the product. And many people don't want to, in any way, support a fetish they see on the same level as things like bestiality, necrophilia, and other polarizing fetishes. There was more I could have brought up, but I think this is more than enough for now. If anyone wants to chat more, I'm down, but I think I've made the points I wanted to.