My remark about having the game happen in a setting where it's legal, was mostly in context of letting the story focus on the social aspects of the taboo relationship, without that extra baggage. After all, I imagine a brother/sister couple won't say (at first) “we can't because it's illegal”, instead of “we can't because what if x found out.” The social aspect always hits first, the legal aspect is more of a practicality, a concern for later.
While I fully agree with that, I also think very few incest games actually confront that objection. They might nurse the "sis/mom/dad/whoever can't see us" objection for a while, but the MC is always immediately DTF and so, fairly quickly, are all the relatives. At which point the inevitable harem becomes, well, inevitable.
I think the core of my objection to this has little to do with incest as the context and more to do with the lack of any actual impediment to the sex that's going to happen. There are very, very few games in which there's a simple "two people decide to have sex" story because that's the opposite of interesting, and if that's all we want about 2/3 of porn depicts two people who decide they want to fuck, they fuck, and everything's fine. Games almost always introduce some sort of hurdle (incest, extramarital sex, the mafia, zillions of deceased fathers) and a fetishistic goal (incest, assembling a harem, banging your teacher, D/s or BDSM, minotaurs, whatever) because the implied difficulty...whether or not it's actually difficult in the gameplay...is a huge part of the appeal of sex that mainstream porn can't always legally depict.
The worst porn I've ever seen is Free Use porn. While it's true that these are actors paid to have sex on camera, and that no matter the script I'm well aware they're
going to have sex on camera, the complete lack of any barrier or impediment is such a turnoff.
I think, the main aspect of the incest kink, is the actual transformation of a Platonic love to a romantic love (which would be why step-relationships still work). And with father/daughter, that of a caretaker being seduced by his charge.
The thing is, I fully understand sibling incest even in a real-world setting. (I say this as an only child, mind.) You're with this person all the time, you hit puberty (and encounter all those surging hormones) at roughly contiguous times, you share rooms and experiences, you've probably seen each other naked or at least barely clothed, other people are scary but you have a reasonable expectation of trusting your sibling, you have age-typical desires, and the other person's right there, feeling all the same things you're feeling. I don't think they usually need a great deal of psychoanalysis save for the guilt/shame/whatever in the aftermath...which is one of the ways
Haley's Story fucked up, because introducing Klaus meant that they did need to interrogate Haley's psychology, and they punted.
I think that most generational incest stories rely entirely on reverse sexual imprinting (a/k/a the Westermarck effect) by dumping someone into a home for the first time...whether that be sisters plus mom, or a father meeting his daughters for the first time...because it's so much easier than the more difficult work of making those relationships work without that particular crutch. I know (very well) about two siblings who had a thing and managed to get past it and are still close (but not sexual), and I know of two father-daughter relationships that happened within solid families...one of which turned out okay, the other of which turned out as horribly as most might expect. The latter is never going to be a game, or at least not one that anyone would play. But the other two might. (There was a very, very brief time when I wondered if Haley's dad might've been a different person than he ended up being.)
(Dating my Daughter falls in to this trap, I think. With the D becoming sexually interested in other people, even seeing sex as a reward for a good deed by a man she's interested in.)
Oh, I have so many problems with DMD...as a game, as a story, and as some sort of exemplar of generational incest. But to be honest, my only problem with Dee & Martin was that, by the time they finally pulled that trigger, a majority of the dev team was against doing it, and so they wrote a wretched caricature of...well, a lot of things. There was a much, much better way to write that whole optional sidequest, and they weren't capable of it. My biggest complaints about DMD remains the ones I've had since early in Chapter 1: Dad is a creep and a groomer, his daughter is an idiot, and none of them are able to write functioning human beings.
Concerning Haley's Story, I think this game somewhat subverts the incest trope in AVN because it'll usually be the male/MC who will pine after his sister first.
Which is one of the biggest reasons I liked it from the start. Haley's the aggressor. (Also the brat.) Her brother goes along until she pushes the wrong buttons, which instigates the whole D/s sub-narrative, but the idea that the male protagonist/MC isn't the perv pursuing his hopelessly naïve sister, but rather the recipient of her interest, was (frankly), hot alongside being fascinating.
And finally, speaking from my own experience, I watch incest porn because it typically features body types I like. I don't think it's far-fetched that this can cause a reverse association, where subconsciously, 'incest' just becomes a synonym for “imagery I can get off to.”
And, frankly, that's probably another reason I struggle with it. I don't really care for realistic MILF bodies, to the extent that some games depict that; I don't mind the huge boobs unless they're truly parodic, but the massive asses are a turnoff. But lolis are a hard limit for me, just as much as real porn or scat. So, for example, I can enjoy
Sisterly Lust (while greatly preferring Bella to Susan), because all those women have reasonable bodies, but I've always had a hard time with
Parental Love, because Ava's simply too young for me. Even un-modded. I doubt I could play that version.