Thanks for the thought out post. I think it is entirely legitimate to enjoy NTR games for the sexual content. I am careful to specify that I refer to people who see NTR tag as the draw to a game. If a player were into 3rd person cuck, that's under the 'cheating' tag, which is different from the NTR tag. The NTR tag, as we know, signifies that it is the player character who is cucked.Decent take and I agree with most of it though I think NTR and cuckoldry have a lot of nuances to it.
(Disclosure - I'm not into NTR nor am I an overt fan of sharing/swinging but I get the appeal to it.)
I think part of playing games depends on your POV you take in playing a game. What I mean about that is if you're coming at it from an invested first person POV you care more about the characters actions since its from a personal viewpoint. Actions and choices you take form a personal relevance and can be more impacting to you since you immerse yourself into the character. From there it's up to your own personal preferences. Do you have a cuck fetish? Do you enjoy the pain and suffering? Does it have to be about the pain? Maybe you are more of the voyeur type and that excites you? Maybe you don't connect with an LI in a game so their actions don't affect you personally? Some of it deals with masochism and sadism tendencies (doesn't have to but for some it does). If you're into personal degradation or humiliation by someone, a game that does that to a MC might be a reason you're attracted to it - hence why a game labeled NTR and has those things in them would be appealing. There's a whole host of reasons or situations or rationales that justify why someone can be "into" that type of game.
Coming at it from the third person perspective you can shelter yourself a bit from the personal feelings or actions of a MC. Maybe you feel more akin to another character in the game and don't identify with the MC (I know people of mixed races and friends who are black who do this with games involving a black character or anybody of color in the game that has a white or Asian protag - for them they have no connection to the white male MC and identify with others). Putting race aside - maybe you're someone of a vastly different age than the MC and that's a reason for your disassociation (there can be a whole host of reasons for the disassociation). You also can live out a little of your sadistic preferences seeing the suffering or anguish of a main character if that's your thing. That feeling of indifference of "welp, this guy's not me so I don't care what happens to them... I'm just here for the sexy action between characters" is a way some can justify it. I think disassociation and not projecting yourself onto a character plays a big role in it for those who play the games with NTR from this 3rd person POV who can also state that on a personal level they aren't into the idea of cuckoldry or being cucked.
That disassociation from a MC's actions because you don't personally feel you're the one involved in the act, but rather another person entirely that you're just watching and/or influencing, speaks more to how you as a player want to immerse yourself in a game. (There's been tons of research on 1st person POV/3rd person POV in game immersion studies -- here's but one:You must be registered to see the links). A lot of male players who play female protag games can take this stance to a degree - the idea where "I'm just following around this person and influencing their actions based on my choices but at the end of the day it's not me personally but someone else entirely".
But again there's always nuances to it - you definitely can take a 3rd person POV and be into cuckoldry all the same (wherein you personally enjoy being cucked). I personally don't seek enjoyment in pain, humilation, cuckoldry, denigration of someone, etc (they can all be the same thing while also being completely different too) but I'm not going to judge someone or make some definitive statement on what's the best way to "get off" haha. People all have preferences, desires, and fetishes - I know what I like and what makes me happy. If something I don't like makes others happy, meh who am I to judge?
A player who takes a 3rd person view of cucking may find pleasure in both NTR and cheating games, not NTR games exclusively.