AN: Takes place in the comicverse. I haven't actually read them, but I know that at the end of season 8, Xander and Dawn are living together in San Francisco, and Buffy lives on their couch. And that's all the info you need to understand the story!
Xander had just clicked off the lamp and snuggled under the covers, when Dawn suddenly bolted straight up in bed and turned to him, panic written all over her face.
"What if I turn into a demon?"
"Excuse me?" Where the hell had that come from?
"A demon, Xander. A nasty, slimy, icky demon!"
He sat up and reached over to turn the light back on. "Dawn. Sweetheart. Why on earth would you turn into a demon?"
"Because!" she shrieked, seemingly annoyed that he was failing to read her mind, "we're happy, and if memory serves, happy equals badness in relationships!"
He almost told her she was being ridiculous, but then he did a quick mental rundown of all the relationships she'd been around. Crap. She kind of had a point.
He reached out and placed his hands on her shoulders. "Hey." Her eyes met his, still wide and panicky. "People are out there, right now, being happy and nothing bad is going to happen because of it. It sucks that you had to grow up on a hellmouth. It sucks that I had to grow up on a hellmouth. In fact, it sucks that anyone, anywhere, has ever had to grow up on a hellmouth. And, yeah, the things you and I have seen, when it comes to happy couples, don't exactly spell encouraging. But it doesn't mean something bad is going to happen." He grinned. "It just means we have to make sure to make enough money to pay for all of the therapy we undoubtedly need."
Her panic seemed to subside slightly, but then she looked down, gnawing on her lip, and muttered something.
"Huh? I didn't catch that."
"I said, what if I turn into a demon because I'm dating you?"
He dropped her arms like they were hot coals, and she folded them defensively across her chest.
"Oh, come on Xander! Is that really such a strange thing to worry about?"
"Hey! I was around for all your relationships! Skanky sub? Giant bug thing, tried to kill you. Impata? Ancient mummy, tried to kill you. That girl from the hardware store in Sunnydale? Evil demon, tried to kill you. Anya? Vengeance demon, never tried to kill you, but tried really hard to get one of us to wish that your penis would shrivel up and fall off, or something. Not the best track record."
Xander gaped at her, trying to figure out how to respond in a way that wouldn't sound completely childish. When he couldn't find one he chose instead to expel, "Cordelia wasn't a demon! And she never tried to kill me! Well, unless you count all the times she tried to humiliate me to death."
"You know as well as I do that she got magicked up with demony bits. It counts."
"I think we can safely blame that on falling for Angel, not falling for me."
Dawn smirked in a way that reminded him she'd spent way too much time with Spike during her formative years. "Counts."
Refusing to accept defeat entirely, Xander lamely offered, "I went out wit Willow in kindergarten."
"Witch. Tried to kill you."
"At least I never went parking with a vampire!"
Dawn cracked a smile. "The monks made me out of Buffy. Of course I went parking with a vampire."
Xander laughed, and suddenly all the tension went out of the room. "You better hope I don't go wake her up and tell her you said that."
Dawn scoffed. "Mean!"
She looked so adorable, sitting cross-legged in their bed, face flushed from giggling, that he had to lean over and kiss her. By the time he pulled away she wasn't laughing any more, and her face was flushed for entirely different reasons. He cupped her cheek.
"Dawn. You will not turn into a demon, okay? I promise."
She took a shuddering breath. "Okay."
Xander turned the light off again, and pulled her into his arms.
"Xander?" her voice was small, almost swallowed by the darkness.
"What if I did?"
"We'd fix it."
"But what if we couldn't? What if it was permenant?"
"But what if-"
His arm tightened around her. "We'd deal."