I think the way taboo scenes feel 'natural' often hinges on the author not treating the forbidden aspect like a neon sign. A lot of weaker entries in the genre just slap a 'step-sibling' or 'boss-employee' label on two characters and then have them jump into bed, expecting the label alone to generate all the tension. That feels cheap and forced. The better approach is to build a world where the transgression is an organic outcome of the characters' specific circumstances, flaws, and desperate wants.
Take a story where a character is deeply lonely after a loss and finds solace in someone 'off-limits.' The sex isn't about the taboo for its own sake; it's a manifestation of that loneliness and need for connection, with the forbidden layer adding a painful, complicated gravity to the act. The scene feels natural because the emotional drive is real first, and the transgression amplifies it. The characters might hesitate, their internal monologue full of 'this is wrong' even as their bodies move together, which creates that delicious, agonizing friction.
It’s that friction—between societal rule and visceral human need—that makes it compelling. When done well, you’re not just watching people break a rule; you’re understanding, on a gut level, why breaking it feels inevitable for them in that moment, even as part of them hates it. The aftermath is where a lot of stories fumble, but the best ones let the consequences linger and reshape the relationship, making the initial transgression feel even more weighty and real.