Ihe, do you mean the suspension of disbelief *can* dull the emotional significance, or *always will*? Because the former I can agree with and the latter I can only say is the opposite of my experience. If anything, I think the juxtaposition between 'make-believe' and harsh human experience can frequently make transgression all the more jarring.
I meant it as *almost always does*, but I see your point. A transgression in SFF can be quite poignant when taking it as an allegory/parody/comparison of/to the real world, but that will depend largely on the context of the plot, and in what specific angle the inner workings of your book world mirror our world, in order to properly blow up the significance of the scene. Also, some people will not allow themselves to engage with/think about taboo subjects, and escapist lit can help them do that without feeling uncomfortable, thanks to the perceived distance. So in that instance, disbelief actually helps to start an internal debate the reader wouldn't have had otherwise.
Thing is, I was thinking of the average reader, not so much the avid SFF fan. For us, suspending disbelief is a non-issue, but not everyone can consistently achieve that state of mind while reading, not without practice. For the average reader, a taboo situation in an impossible setting (ie, being in the centre of the sun, in the cave of a diabolical talking dragon, or being married to a bioengineered Space Marine fighting for the Galactic Human Empire 5000 years from now, etc) is tasted differently because the setting and incredible situations themselves can sometimes overshadow the scene (ie, "Holy crap! I'm inside a black hole fighting a cyborg mage, who cares about incest right now?!").
Whatever the case, I believe suspension of disbelief is the main predictor in the emotional connection to a book. The more you need of one, the less you'll see of the other. The trick is to make belief and disbelief irrelevant for the time it takes that single important scene to develop. Let me try to explain:
Just knowing that the story isn't real is enough to create cognitive and emotional distance between story and reader (all fiction is make-believe, I know, but SFF is considered to be doubly so). Now, if you go all the way by wilfully increasing said perceived distance, taking the idea as far from reality as you can, make the reader aware of such distance, and then show the reader how the transgression is still relevant, then the transgression can become a symbolic interpretation of the taboo, as it is so far off from known reality that it sheds its physical body and cuts all ties to belief and disbelief alike, becoming an ethereal element in the mind of the reader. Only then is suspension of disbelief made irrelevant, the cognitive barrier goes down, and the full emotional brunt of the scene comes through. This is possible once you force the reader to stop comparing your story to the real world, and help him/her perceive certain things in a more symbolic form, which is a form that can do no wrong to their cognition. That's why I think that the more extreme the SFF, the more significant taboos can be in the story. So I find that the impact of taboos follows a "U" shaped progression through a graph that goes from "realistic" to "way out there", dipping in the middle, where the more subdued SFF is, and then picking up again into the more ludicrous section of the library. Anyway, that's been my experience. The reason why, in my opinion, "partial" SFF has the hardest job of dealing successfully with taboos is because it tries too hard, picking up flight into SFF heavens while being constantly jerked down by the chain that ties it to the real world. The story can't make up its mind therefore neither can the reader, and this confusion/ambivalence can get in the way of an emotional connection. But once you forgo attempting a resemblance to "reality", then the reader can jump on-board, suspension of disbelief becomes the default background state, and the reader no longer thinks about it or subconsciously analyses the emotional distance. So, for me, taboos work best in "very real" scenarios, or "very strange" ones.
So, to summarise, suspension of disbelief is the primary predictor of emotional connection to scenes, IMO. It is the way a writer handles that suspension that makes or breaks a poignant scene though (so there is some wiggle room in my theory
--THEORY, that's what it is. I'm not being categorical in any way). And obviously, the reader's ease to achieve suspension of disbelief plays an important role too.
Don't know if I explained myself even remotely well, but this felt like trying to put into words a very complex feeling (as a reader), more than an intellectual argument (as a writer). So apologies for the possible "huh?"s this might raise
.