Lately I read Louise Perry's book, and listed to a bunch of interviews by her. She's really smart and I think she's right about a lot of things.
I think to some extent the young women now who are really into choking or whatever… I think that in some sense what they're doing is they're interpreting the violence as evidence of them being desirable, and they think their boyfriend is demonstrating passion for them…
But I think there's a kind of mutual incomprehension, because I don't think that's actually how the men are viewing it. If you look at the strangulation porn or similar that's intended for a male audience, or you hear testimony from men who have struggled their partners, they don't generally see it as a sign of passion. They tend to see as a sign of contempt.
Louise Perry interview, "The Sexual Revolution And Its Discontents"
In incest fanfic, written by and for women, the transgressing of the incest taboo is a dramatic display of burning desire.
Incestuous desire in fanfic is kind of the concept of being friends before being a couple taken all the way to its logical extreme. Fictional incestuous desire is rooted in knowing one another inside out for years beforehand. It's rooted in fervent admiration and adoration that's busting at the seams of acceptability. It's also extremely monogamous; a common trope is seeking an acceptable partner to transfer these sublimated desires onto, but being unable to do so because the beloved is irreplaceable. Fictional incest is extremely committed; there is no going back on this, they're effectively married since the first kiss.
This is what incest symbolizes within fanfic written by and for women.
In real life, I think the most common paradigm is that targets of incestuous desire are are chosen because of simple opportunism. If you're looking for someone to sexually abuse, family is there, conveniently on hand. And the trust within families means they don't have the same kind of safeguarding measures that public institutions have. I don't know a lot on this subject, but what little I've read suggests that this might really be all there is to it. Real life isn't about symbolism. Opportunism is a much more down-to-earth, real-life kind of motive.
But I don't think it ends there. I don't think there are just 2 paradigm of meaning for incest. I think literary-incest-framed-as-bad might have it's own set of symbolism. What that is, I'm hard-pressed to say. I've yet to find an articulate anti who's willing to lay it out for me. I would be deeply curious if someone ever was. In the meantime, I'll make some tentative guesses:
Like in women's-literary-incest, there's the idea of likeness, mirroring. While that genre might frame loving someone who is like you as a vehicle for learning how to love yourself, this genre instead frames attraction to someone like you as narcissism or arrogance. Incest in insular in a claustrophobic way. I have seen it posed as promiscuous: disinterest in kin is seen as the most baseline of all standards, the absence of it indicates the absence of all other standards as well.
And I'm not sure it ends there. I'm going to guess that content made by and for men where incest is framed as desirable is again different. This is the one I know least about, and the one I'd be most afraid to learn about.