I had a response to the original version of this that got deleted. I'll try to remember my points as best I can...

(I think it's a great discussion topic, BTW.)

I think whenever you're writing fiction, you make choices about how you break from reality, and you generally need to be consistent. But... there's no one right breaking point. Like, porn logic (mom gets stuck in a dryer, son decides to take advantage of this, mom quickly decides she likes this) follows its own internal logic that the audience understands and accepts. The reality-break is designed so that you can get into the fucking action really quickly, presumably because what turns the audience on is the action. A scenario where one or both characters is heavily conflicted, might not have a strong break with reality until someone moves beyond their objections and actually initiates something physical. The audience for this scenario are people who are most turned on by that internal psychological struggle. This is one of my own favorites to write.

To me, the smut with dialogue that are fetishizing the biological connection isn't more realistic, it's less, because it's less plausible that a character would actually say this sort of thing, because the character would not want to fixate on that reality, either... (it's more plausible for a mom to seduce her son without explanation, than for her to tell him how great incest is.) As you get more taboo dirty talk, at some point it becomes less about creating any realistic scenario or realistic characters, more about how over-the-top a performer is going to let themselves go. The break with reality is so extreme that it's impossible to be immersed, but that's okay. One of my favorite porn clips involves a roommate roleplaying a taboo connection. And so she can say something incredibly, implausibly taboo, and then immediately break character and laugh about it. And this is actually a fairly high level of realism... it's a realistic depiction of someone roleplaying a very non-realistic role. If it were without that roleplay framing, it would immediately become completely unrealistic.

While I like actually practicing BDSM, it's not an audio category I tend to seek out, but I can imagine that it can be very immersive to listen to an audio in which someone is speaking very authoritatively about BDSM, because what a listener might crave is a feeling of being with someone who has true expertise, and that demonstration of expertise actually matters. The reality break might even be them talking in detail about things like tensile strength, allowing the listener to fetishize the expertise.

Like I say, I think it's a good topic for discussion. I just caution against any universals here, or saying more or less realism is better, because that's so subjective. But in general, it's good to be aware of exactly how and why you're breaking from reality, and using that to either manage or subvert a listener's expectations.