The importance of having an MC really comes down to what kind of story you're trying to tell. It's impossible to tell a story without some form of perspective character, even stories in third person has "main characters", there's no getting around it without getting very avant garde and esoteric. For the most part porn visual novels stick to a single main character because the fantasy or experience that the game is trying to sell is that of self-insertion and usually the building of a harem, or for the perspective character BEING the experiencer. The latter is especially prevalent in games with a female protagonist, as for the most part it seems that the way the narratives frame the kinks and sexual scenes are as things that happen to the protagonist. Even the act of chosing to engage in sexual activity or perversion is something that is being done to female characters, because in those games the kink being sold is that of corruption. With games like this, the use of third person or other perspectives characters tend to be one of scenes to act as often either displays of how the mcs actions are affecting a love interest (like a mother character catching a male MC masturbating to someone's underwear and is overcome by lust or some such and absolutely can't look away), or how another characters actions are about to affect the mc (such as some perverted street thug checking out hot female mc and is about to mug them and possibly more).
Whether or not this style of framing a story is more enjoyable for people is debatable. It's certainly an easy way to write a story, and useful to catering to certain kinks or desires. But it is limiting in the kinds of stories and experiences. A good example of this I would say would be the game Seeds of Chaos, which while primarily is from the point of view single character, does have EXCELLENT scenes from the perspective of his wife. While it does lean towards the NTR side of the spectrum, there's enough of a polyamory spin on choices you can make that I particularly enjoy. By having multiple, or at the very least by having a secondary perspective, even if limited to a few spots in the game, the story and kinks that the game is then able to give to the play are so much richer and nuanced, though a lot of that comes down to the skill of the writing rather than just the use of perspective itself.
In regards to what Person the perspective is in is another matter. You claim porn is in third person but you can't quite apply first second and third person to film. There's a level of cinematic theory I could go into that I am NOT well versed in enough to explain, so let's just leave it as in regards to written stories, first second and third person are a matter of language: "I did this" "you did this" "they did this". This is all relevant to visual novels and narrative based games as it frames how the story interacts with the player. First person is something being presented to the player. This is your Phoenix Wright kinda perspective. A story about a specific character experiencing the arc of the narrative, where the game lays around the choices, not within the choices, e.g Danganronpa offering you the choice in what characters to get close to and have free time scenes with, but the game is in the trials. Second person is agency and experience being given to the player. This is your Slay the Princess kinda games where your choices matter and the experience is aimed at you. This is a perspective very common with visual novels, especially self insert porn games, to the point of almost being unnoticed. Third person on the other hand can be used to build distance between the player and the characters. In my experience this perspective can occasionally be found in trainer-esque style porn games like Tara's Liberation, where the focus is on corrupting a female protagonist. This particular person isn't all that common, and in truth doesn't necessarily lend itself to the kind of perspective switches you're describing. It's all top easy to stay locked into a single characters perspective even when the writing is in third person. A game that you could consider a third person game in the sense that you want, i.e a game where the perspective shifts between multiple people, would probably be something like Detroit Become Human. A game with multiple protagonists, who the player rotates between through the story and who's decisions affect.
As for porn games that do this kind of perspective rotating... Couldn't tell ya. It would be nice to experience a standard incest game set up from the various perspectives of the family members, but I think the reason why there aren't games that do something like that is because they just ... Don't see a need for that. Or because it's easier to stick with the simple thing and because the core hook/experience of the game doesn't necessitate the switching out of perspectives. It's just easier to stick to one character for most of the time.
I dunno. Sorry for the essay I just had a lot I wanted to say
