Man, I've thought about this a lot since a friend got into an argument about whether some of the stuff she reads counts as 'real' romance or just smut. The distinction gets blurred, but I'd say literature erotica puts the physical, psychological, and often transgressive journey of desire itself as the central narrative engine. A mainstream romance novel uses intimacy as a crucial component in a story whose ultimate goal is the emotional, committed relationship—the 'happily ever after' is non-negotiable. The spicy scenes serve that goal. In erotica, the relationship is often the vehicle or the context for exploring the depths and edges of desire; the 'happily ever after' can be ambiguous, or the point might be the transformative, sometimes destructive, power of the encounter itself.
Take Anne Rice's 'Sleeping Beauty' trilogy versus, say, a standard historical romance. Rice's work is unapologetically about the exploration of a specific, intense BDSM-centric world. The character arcs are about submission, dominance, and awakening, not about securing a monogamous marriage by the end. The prose dwells on sensation and internal conflict around desire. A mainstream romance might have similar power dynamics, but the narrative will consistently bend them toward mutual understanding, healing, and a socially-sanctioned union. The pacing differs, too—erotica can sustain a higher, more consistent temperature of tension because the release isn't solely reserved for a final emotional commitment; it's woven throughout the exploration.
It's not just about more graphic sex, though. It's about intent. Erotica asks: what does this desire do to a person? What lines does it cross? Romance asks: how does this love save or complete a person? The answers can overlap, but the primary question dictates the genre's soul, I think. Some of my favorite books live in the messy middle, honestly.