Disclaimer: I don't own the Avengers, just the ideas for the fic.
A/N: Uh...okay, so this is my first time trying my hand at writing about sex in a fictional and descriptive way. I think I do the deed some justice (haha), but mostly I'm just hoping this sounds all right without coming off as cheesy and laughable.

The bedsheets were tangled and the comforter had been kicked to the floor. Their bodies were inseparable, rocking and swaying in rhythmic unison. Their mouths were hot with desire, kissing and nipping at each other's skin.

"Let's make our vows," she said through gritted teeth as she thrusted and rotated her hips on top of his.

He closed his eyes in euphoria, drawing in a deep breath before flipping her onto the bed and underneath his body. He moved with a sudden agility that both fascinated and frightened her. He pressed his forehead to hers, locking her eyes in an icy gaze.

"I promise," he said as he pressed himself further inside her, making her gasp. "I will never..." Another thrust. "Love you."

His voice was a low, throaty growl, simultaneously mischievous and aroused. She smirked at his words and ran her fingers lightly down his spine. It never failed to send waves of chills throughout his body, making his entire being shudder. She flipped him back over with surprising strength. Once again, she sat atop his hips, methodically rotating her own. His hands rested on her thighs, squeezing the gentle but firm flesh and then releasing.

"I promise," she began her vow. Then she bent down to whisper in his ear, "that one day, I will kill you."

Her breath was hot on his ear, and then came the wetness of her tongue lightly flicking his lobes before tickling the inside of his ear. He groaned and felt himself losing control. His eyelids fluttered, unable to stay open and hold her gaze. Every muscle in his body contracted before releasing in a burst of pleasure and energy. Spasms rocked his body as he gasped for air. Seconds later, he felt her pressing against his pelvic bone, saw her mouth fall open, heard her raspy moaning. He reached his hand between her legs to assist in bringing her to a similar climax. Her entire body shook as she emitted a sound between a moan and a growl.

She detached herself from him, collapsing onto the cool sheets of the bed. Their skin was slick with sweat that seeped into the soft cotton threads. She lay on her back, eyes closed, trying to slow her breathing, a pleasured smile still on her lips. He watched the rise and fall of her chest as her breathing pattern returned to normal.

The words they had said to each other rang in their ears. They had initially started their affair with their warnings before any action had been taken, but then their menacing threats had somehow made their way into the bedroom. It was becoming almost like a tradition to utter the words every time their bodies merged into one. It was a reminder; a way to remember that they were enemies, not lovers. They had no emotional connection. They were not supposed to be attached to each other. They were simply carrying out the most basic animal need. No strings. No emotions. Just sex and threats.

When she had regained her composure, she rolled out of bed and gracefully sauntered over to the bathroom. She grinned as she grabbed a towel from the cabinets and threw it at him before closing the door. He took that as his cue to clean up and prepare to leave. He took his time, not wanting to wipe away the traces of her body, her scent, her essence. She quickly reemerged, fully dressed, not looking the least bit flustered, save for the rosy glow of her cheeks.

"Loki," she said, her voice quiet but stern. "Until next time." She spoke as if they were in the middle of drafting a business contract, as if the sexual activity that had just occurred was only a daydream, a foolish whimsy. If they remained professional, it wouldn't become personal. She nodded at him and headed for the door.

"Natasha," he called to her just as her hand reached the doorknob. She turned to face him, a calm but inquisitive look on her face.

"Do you really intend to kill me?" he asked mockingly. It was a rather difficult promise to undertake, killing a god.

With a sly grin, she turned the handle and opened the door. "Do you really think you'll never love me?" she shot back.

They stood there staring each other down, feeling smug with the satisfaction of calling the other one out on their respective bluffs. Try as they did to sound convincing, they knew their vows were nothing but lies.