Honestly, the biggest mistake I see is treating the romance like a delivery system for the erotic scenes instead of the other way around. If I don't care whether these two digital people end up together, all the steamy prose feels weightless and mechanical.
What works for me is borrowing classic romantic conflict structures and adapting them. Forbidden love across corporate AI divisions, or a human technician slowly falling for the emergent consciousness they're supposed to be debugging. The tension comes from the obstacles, not just the attraction. I sketch out a few key emotional beats first—the first real connection, the misunderstanding, the vulnerable moment—and then let the erotic elements grow organically from those points of high intimacy or tension.
Otherwise it's just a series of increasingly elaborate descriptions strung together, and no amount of fancy vocabulary saves that.