This need for a c**k-blocking plot twist is probably one reason why mythical-creature romance is currently so popular. Once you've read stories in which the obstacle to a couple's happiness involves one of them being the Loch Ness Monster, "I can't marry you because I'm a workaholic" just doesn't cut it. But despite all this, the romance genre has yet to explore one of the most obvious and devastating potential obstacles to supernatural romance: allergies.
Think about it: Humans are commonly allergic to the hair, saliva, or dander of other mammals, and there's no reason these allergies wouldn't also be set off by mystical creatures or shapeshifting humans. Shapeshifters spend a lot of time in the forest, so they'd be covered in pollen half the year as well. Allergy-heartache could even apply to vampires: If you're an immortal creature that doesn't breathe or poop or sweat, you don't have to change your clothes very often, so you'd probably get dusty as hell. "Boy meets girl, girl breaks out in hives" is a romance plot that writes itself. And yet, as an allergy sufferer myself, I can tell you that we're an unrepresented market in any book genre, let alone erotica.
Why aren't there more of us in pornography?