You'd be surprised how many readers overlook the 'Beauty and the Beast' collection by Emma Alcott when discussing this niche. It's not a traditional wife swap label, but the setup where partners are essentially 'exchanged' into rival aristocratic households as part of a political or magical pact hits every note of romantic tension you could want. The forced proximity and the slow, agonizing realization that the person you've been bound to isn't the monster you imagined, while your original spouse becomes a distant memory wrapped in duty, is executed with a delicate, anxious hand.
The erotic elements aren't gratuitous; they serve that tension, born from stolen glances in gilded hallways and whispered conversations under the pretense of negotiation. I found the third book, 'A Crown of Thorns and Silk,' particularly adept at making the reader feel the protagonist's guilt and burgeoning desire as equally powerful, warring forces. It's less about the physical act of swapping and more about the psychological trade, which, for my money, creates a far more lasting and heated effect than more blunt narratives.