It just sort of... happened.
It was cold and they were tired and it was too much effort to drag themselves back to the hotel after the job was done. Lucy's spirits got the hut and wood together and Natsu stacked the fire so that it would burn all through the night. It was still cold, so they moved their sleeping mats and blankets side by side and huddled together to keep warm. Lucy kept jolting awake to the sound of Natsu shuffling around. Then his hand and foot pushed against her in his sleep and she passed out.
She woke up at grey dawn cozy and drowsy to the feel of him behind her with his hot breath on her neck and his arm around her waist and his leg tight around hers.
He had wriggled into her bedding.
It took her a couple of seconds for that cozy feeling to turn to something more complicated... and mortified.
Especially since she could feel a half-interested boner nudging her in the rear.
She was still frozen, staring wide-eyed out through a crack in the thin wooden-slat door when she heard Natsu's breathing change. He made a quiet sigh and she could feel his muscles go from lax to awake. He didn't move away. Instead, he shifted so that his hand was above her head. She felt light tugging at her hair as he flipped it idly through his fingers. Then he leaned closer to her neck, so close that she could feel the barest trace of his mouth, and he inhaled deeply.
Lucy felt her toes curl and pleasure unexpectedly coiled through her.
Natsu froze.
Lucy blinked frantically.
Then she did something.
Something that she must have proved that she was not fully awake, because if she had thought about what she was doing, she would have... Well, she would not have done what she did. But she was sleepy, and cozy, and happy...
And she wanted.
She pushed her rear harder into him.
He inhaled sharply, going completely tense behind her.
She didn't move.
He pressed his warm, open mouth to her neck and dragged his teeth across her skin. She shuddered and a small sound escaped her.
It didn't take long after that.