That question really hits on why the subgenre stands out. 'Married to a monster' as a premise starts by removing a lot of the traditional courtship dance. There's no pretense of him being Prince Charming, so the negotiation of power, safety, and consent is thrust front and center from page one. It’s not will-they-won’t-they; they already are. So the tension builds around whether genuine affection, or even a form of love, can emerge from a foundation of fear, coercion, or alien biology.
I find the best stories in this space use the 'marriage' as a crucible. It forces the human character into sustained, intimate proximity with something terrifying, which gradually chips away at the monstrous facade. You see moments of unexpected vulnerability, like a beast that hunts for its mate to provide food, or a demon who meticulously guards her sleep from nightmares. The darkness comes from that initial imbalance of power and the inherent danger, while the romance grows from finding humanity—or at least, a code of honor—in the inhuman. It’s a redemption arc for the monster, but one where the 'happy ending' might look profoundly different from a normal human relationship. The whole dynamic forces you to question what you’d accept for love, or protection, or survival, which is why it feels so potent in the darker corners of romance.