I'd say it's more about it being a porn game where the the MC is an extension of the player (we make decisions on the MC's behalf), whereas in regular porn you're just watching characters.
Personally, I'd take porn out of the equation entirely and just put the whole thing entirely down to player agency.
When you're watching a movie, you're watching a story play out for other people. People who are not you, whose lives you have no control over, and who you are only partially invested in. But when you are playing any game -
even if you don't strictly self-insert - you are actively taking control over the protagonist, which almost automatically invests you in their decisions and outcomes far more heavily than just watching a movie or show.
Whether it's a porn-heavy AVN or a game like Mass Effect, making choices for the character makes that character more real to you - and thus you're more likely to react more viscerally to things that bother you on some level. Shooting the bad guy who's been screwing us over for hours worth of gameplay is
far more satisfying in the moment than just watching some other guy beat up
his nemesis. It's why so many people want to push the idea of games-as-art/interactive storytelling in the first place.
For a lot of people, unavoidable NTR, sharing, or otherwise knowing that the LIs you're pursuing are with other people (willingly or no) triggers a deep revulsion, because regardless of whether or not you're playing the MC as if they were
you, you're still seeing the world from their perspective and putting yourself in their shoes to some degree. So it
feels much more like it's happening to you than it does when you're just watching a movie where it happens to someone else.
I can watch a movie where the protagonist's girlfriend is kidnapped, gangraped, and murdered - I would
never want to play a game where it happens (to "my" girlfriend). I can watch porn where a bunch of guys run a train on someone's wife, but I'd never want to play a game where I'm put in that guy's shoes. I can watch Old Yeller, but I'm
never going to want to play Old Yeller: The Game.
Ultimately, the last thing most people want from a form of storytelling that exists more or less solely to empower the audience and allow them the freedom to shape the narrative is being made to feel absolute
helpless (unless that's actually what you're into, vis-a-vis bondage dom/sub relations etc).
I find the ways some people limit themselves based off an imprecise list of tags sans context instead of engaging with a work on its own merit peculiar
The thing to remember is, ultimately, people understand their own comfort zone far better than you ever will.
There are people who literally cannot play games like The Last of Us, because watching Joel's daughter die is their absolute breaking point (especially if they have young daughters themselves in the real world, because they empathize too strongly). There are people who have trouble playing any video game with realistic-looking spiders because they have extreme arachnaphobia and even games like Skyrim skeeve them the fuck out. There are people who have an extremely hard time playing Japanese visual novels at all (even the non-sexual ones), because the Japanese trend of extremely passive doormat MCs can be
extremely frustrating to players who want to be proactive. There are people who love incest in their adult games, and there are people who
loathe it. Some people are going to be fine with watersports/golden showers/piss drinking/etc in games, and other people who will feel viscerally ill or just really, really grossed out.
And that's fine. Everyone is going to have their own personal turn-ons and turn-offs, or things that bother them so much they simply can't just play through and ignore it. Some people will avoid NTR like the plague, others will actively seek out games with NTR because it's what they want. Neither is
wrong - they just want different things.
The entire
point of content tags is to clearly indicate "This is what this game contains", so people can make an informed choice whether or not it's content they want to engage with. Because no one piece of media or work of art or story is EVER going to appeal to every human being who has ever lived. We ALL choose which media to consume and which media to ignore based on our innate preferences. Someone who hates slapstick humor is never going to watch The Three Stooges (or appreciate it even if they do).
It doesn't necessarily even
matter what the context is - a writer can feel they're telling a brilliant and compelling story that absolutely requires the main protagonist to be raped in a 20 minute long scene, but if the player is sitting there looking at the screen in disgust, pissed off because they literally cannot do anything to prevent it from happening, they're not going to
care what message you're trying to convey or what story you're attempting to tell. They're going to quit the game, delete the file, and probably complain about how terrible your game is on whatever forums they frequent. In interactive fiction (much like in tabletop RPGs), the writer has to be willing to cater to player choice - player agency - and be willing to bend in places where players refuse to go. Or the writer needs to accept that many players may simply opt out entirely.
At most, we can argue that tags
here often aren't all that
clear (which is why you get a lot of questions like "is the group sex/futa/content skippable?" in threads here, or questions like "Is the NTR someone cheating on the MC, the MC sleeping with other people's partners, NTR between side characters, or something else?"), but once someone has made an effort to get a better feel for what the tags actually mean (either by asking in the forum, reading reviews, or by some other means), they're perfectly justified in refusing to play games or engage with narratives that don't appeal to them or which touch on some subject or fetish they're not comfortable with.
I don't really accept the logic that "In order to judge a work you have to experience it in its full context". Nor do I really buy the argument that "Art is supposed to provoke an emotional response, therefore if you're disgusted the art is doing its job and you're obligated to like it". Especially in cases where the art is also intended as entertainment, the audience more than has the right to say "Yeah, that's not for me, thanks."
Which is not to say that such art shouldn't
exist - my not wanting to play a specific game doesn't mean that other people shouldn't be allowed to play and enjoy the game.
Now if you'll excuse me, I'm off to try and find more games with female protagonists, exclusively lesbian relationships, mostly vanilla sex and romance, absolutely no NTR or sharing, and a bit of hand-holding.