An Incubus is a sex demon, plain and simple. They feed off of the sexual moods of humanity. But once humanity found out the Incubi existed, a big war started to get rid of either one species or the other. But there's a problem. Succubi (female counterparts of Incubi) and Incubi need the sexual energy of humans to fuel them, and the men of the human race are dying very fast due to war depleting resources, and everything went to the women. Without men, the human woman can't reproduce and therefore can't supply the demons with the sexual energy needed to keep the demons alive. So some sort of alliance has to be made, between the king of the Incubi and the queen of the humans. An alliance that requires marriage.
Now, I've had too many people asking ridiculous questions. Here is the plot but stripped:
There's a male character, the Incubus king.
There's a female character, the human queen.
Obviously there will be challenges in this arranged marriage because the humans think Incubi are disgusting, and inappropriate, and just much too uncivilized. They aren't, but you know. Humans can be morons.
MOST IMPORTANTLY:
This is set in a Steampunk kind of setting. Women wear dresses with corsets, men wear trousers and poofy shirts and weird vests, and there are advanced machines but no over-advanced weaponry. The most advanced tech is low-tech plumbing.

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It’s the year 2285, and the world is in turmoil. The U.S. Has been shunned by the rest of the civilized world and has become a dictated union. Global warming, world wars, and economic collapse have turned the States into one of the most dangerous places to live. Only 1% of the population is wealthy enough to have homes, cars, decent food and clean water. The rest of the country is living in half-homes and has to scavenge every meal.
This dictatorship locks away dissenters, the sick, the old, and the mentally unstable in concrete prisons underground. The food is poor, the living conditions are nowhere near humane. One girl among these prison outcasts is treated especially terrible. At a very young age, she developed a power that made it impossible for anyone to touch her skin. The government didn’t know what to do with her, so they tossed her in prison to let her rot. Now, ten or more years later, the government finds that they need information about a certain Resistance group. They can’t torture it out of the prisoners in any traditional way… so what if they use her?
They send their young, up and coming general to collect her. He enters her cell to find a mad creature, incapable of speech and the most basic human interaction. He thinks her a beast, a monster for her curse and the job she’s given. But will that change in the next few weeks as he wipes away the grime and teaches her how to be human again?

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Girl: You wake up in a castle, and you don't know where you are. There are so many things out of place, and your memory is blurry, so you really just have to connect the dots. It's not that hard once you notice a few key things: Your clothes are gone, replaced by terribly revealing underwear. Oh, and there's a man sleeping beside you. After learning that you've been gifted to this pervert as some sort of toy, you rebel as much as possible.

Guy: You're the Crown Prince and you're soon to be crowned as King. Your living relatives decide you're not humble enough, not kind enough, or not living up to their standard of who you should be. Basically, you're an ass. So they give you a 'companion' to do with as you wish, hopefully to teach you how to be kind to others. Whatever. She's just another slave, isn't she? But it turns out she's so much more than that.
(This plot is actually much more in depth than it looks. Maps, world history, killer characters and stuff. Pretty awesome.)

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Girl: Your parent(s) die. Filled with grief and anger, you are sent to live with your uncle/grandma/some other relative in a tiny town in the middle of a ridiculously dense forest. You are told never to go into the forest at night, and especially not alone. But filled with rebellion from your parent(s) death, you go in there anyways. You run into a man, a man who has the ability to turn into a wolf. He kidnaps you, and shows you his pack. Your world is turned upside-down by this shocking revelation. You're being pulled and tugged in a million different directions, and through it all you fight through your growing attraction- and fear- for the man who took you, even though he's more beast than he is human.

Guy: You're a werewolf. Not the bitten kind, the born kind. Your pack lives in the forest surrounding the town. There's a treaty that basically says that the werewolves will leave the townspeople alone and vice versa. But if a human enters the forest, they belong to the pack. One night, while you patrol, a new girl to the town enters the forest and you claim her before your pack mates. You show her your pack, keep her safe from the harsher wolves, the ones that dislike humans. Along the way, you discover for the first time exactly what it means to be human.
Also, I have a thing that must be explained.
You know the inner voice everyone has, like a conscience that tells them right from wrong? Well, werewolves have that too, but theirs is broken. They don't have an off switch, and their inner voice is angry, horny, and slightly violent. It's also very persuasive. Depending on how a wolf is raised, that voice can either be tamer or wilder. When the inner voice is wilder, it's harder to ignore, thus creating a werewolf that is moody, easily frustrated, and a bit bipolar. Someone who is harder to control.

Important stuff:
These werewolves are, for the most part, just human beings that can shift into wolves. They don't have an extended lifespan or slowed aging. They are slightly stronger, but not to an insane degree. Some of them have a strong enough sense of smell that they can scent things a few miles away, or scent moods and emotions. This is rare, though. They just look like really big, but otherwise normal, wolves.

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The world has fallen to ruins. Very little people have survived. People can't remember everything about themselves. They're all lost, and confused, trying to survive. Day by day each person goes to find their own Pieces. Each person's Pieces eventually become an object that will completely restore their memories. These Pieces are hidden throughout the world, and each person's Pieces look different. Some people are out to destroy other people's Pieces, or help others find them while they look for their own.
This has been immensely successful for me in the past
So this would be a good few years after the apocalypse.
There are little towns and settlements.
To be clear, let's say that everything short of zombies and aliens destroyed most of humanity. Earthquakes, tsunamis, biological warfare, the whole shebang. Also, no nukes.
The way I have done this in the past is that the male character is a bounty hunter, or a mercenary. He's being paid to either kill or capture the female character for whatever reason. The world was inherited by psychopaths, so they might just want her because she's pretty.
Obviously, he finds her, and stuff happens.

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Girl: You're a princess. In your kingdom, magic is just barely tolerated and your father is trying to get rid of it. But you yourself possess magical powers. You've been kept away from people your whole life, an embarrassment to your royal family. One night, you run away. You travel a nearly impossible distance, even across an ocean, to find the son of a powerful sorceress who you believe can teach you to control your power. But what if it turns out he doesn't want to?

Guy: Your mother was the most powerful sorceress in the world. A true legend. And because she was the best, she raised you to be even better. No child of hers could be anything less than perfect. She treated you cruelly, punishing you with her magic when you made mistakes or refused to learn. You've come to despise all magic-users and magic itself, causing you to never use your own powers if you can help it. Then, a girl comes to you, desperate for your help. She's so young and naive. She won't survive on her own if someone doesn't teach her how. But how can you teach her to control her powers when you hate yourself for having the same gifts?

Girl: You wake up in a castle, and you don't know where you are. There are so many things out of place, and your memory is blurry, so you really just have to connect the dots. It's not that hard once you notice a few key things: Your clothes are gone, replaced by terribly revealing underwear. Oh, and there's a man sleeping beside you. After learning that you've been gifted to this pervert as some sort of toy, you rebel as much as possible.

Guy: You're the Crown Prince and you're soon to be crowned as King. Your living relatives decide you're not humble enough, not kind enough, or not living up to their standard of who you should be. Basically, you're an ass. So they give you a 'companion' to do with as you wish, hopefully to teach you how to be kind to others. Whatever. She's just another slave, isn't she? But it turns out she's so much more than that.
(This plot is actually much more in depth than it looks. Maps, world history, killer characters and stuff. Pretty awesome.)