A few things I've noticed about erotica tropes after reading more than I probably should:
Forbidden desire is the oldest trope in the genre and probably the only one that's truly load-bearing. Every other erotica trope is, in some sense, a subcategory of it. The teacher, the stranger, the monster, the ex. They all get their charge from the same source: the gap between what the character wants and what they're supposed to want.
Power dynamics only work when both sides of the equation have something to lose. A dominance scene where the submissive character has no agency isn't tension. It's just choreography.
Anne Rice wrote The Claiming of Sleeping Beauty under the name A.N. Roquelaure while she was already famous for Interview with the Vampire. The fairy tale framework in those books does something specific: it gives the reader a structure they already trust. You know how a fairy tale moves. That familiarity becomes a kind of permission slip to engage with content that would feel riskier in a realistic setting.
The stranger trope, the anonymous encounter, gets its heat from the same place that first-person essays get their honesty. When nobody knows your name, the performance drops. Characters say what they actually want. The anonymity isn't the fantasy. The directness is.
Forced proximity in erotica is forced proximity in romance with the subtext made text. Same cabin, same snowstorm, same slow collapse of personal space. The difference is that erotica doesn't have to pretend the characters aren't thinking about it from page one.
The teacher/student trope survives every cultural shift because it's never really about instruction. It's about the moment when the person with authority lets the mask slip, and the person without authority realizes they had more of it than they thought.
Ruby Dixon's Ice Planet Barbarians treats alien biology with such earnest specificity that it circles past absurdity and lands on genuine curiosity. The resonance purring, the mating bond, the anatomical differences. She doesn't wink at the reader. She commits. And that commitment is what separates erotica tropes that feel like a gimmick from ones readers come back to for fifteen sequels.
E.L. James put a literal contract in Fifty Shades of Grey. Whole scenes of negotiation over hard limits and soft limits. The contract reads like a plot device, but it functions as a trope in disguise: it turns the power dynamic from something the characters fall into to something they build together, on paper, with a pen. The consent becomes its own source of tension.
Voyeurism as a trope works because reading erotica is already an act of voyeurism, and the genre knows it. When a character watches from across the room or through a cracked door, the reader's position mirrors theirs exactly. The trope collapses the distance between audience and character in a way almost no other genre move can.