He kissed her out of nowhere. One minute he was testing her ability to spot his lies, the next his lips were pressed to hers. She wondered if this was just another one of his tests. For a brief moment she thought about pushing him away, yelling at him and demanding an explanation, but then she realized that whatever the reasons might be - she might not get this chance again. When his fingers brushed her clavicle, she heard a small moan echo in her ears. It took her a moment to realize that she had been the one to produce the sound.

It wasn't as if she had never thought about this - she definitely had, more than once. But even in her wildest fantasies, her fingers curled in the sheets and her tongue pressed tight into her teeth, she had never honestly believed that it could come true. That is - if this was not just another one of his unorthodox tests.
She had considered for a while that he was a person who, due to his extraordinary intellect and complex unwonted behavior (which was a nice way to phrase 'social ineptitude'), would probably never be able to maintain a meaningful romantic relationship with another person. However, she had never doubted that he was capable of deep feelings - it had been evident in the way he cared not just about Irene (however misled those emotions had been) but also about his friends - whether he liked to admit it or not. The sexual components of his love life had been even easier to analyse. In fact, he had flaunted his sexuality in front of her from the first day on - and even though he'd played it off as a physical necessity, she knew as a doctor and an open-minded woman that there were many easier and cheaper methods to control his bodily needs if he did, in fact, not find any pleasure and stimulation in sexual intercourse. The way he had referred to sex with Irene - the woman he had believed Moriarty to be, before the truth had been revealed - had proven to her that if the circumstances were right, Sherlock could very well have a happy and fulfilling relationship just like anybody else.

It stood to reason that, although he would very much like everyone else to believe it, Sherlock Holmes was not a machine.
Neither was Joan Watson.

When his fingers slid into her hair and held her loosely, she brought their chaste kiss to the next level. And in the midst of the fireworks that followed the quick touch of their tongues and lit her body and mind ablaze, she realized that he was holding back. If he hadn't been the one to start the kiss, she might have questioned whether he was rejecting her. And if she didn't know Sherlock better than that, she would have guessed that he was scared. He was clearly very affected by the kiss. The veins of his wrist pulsed heavily against her neck, his breathing speaking volumes to her newly fine-tuned mind. But it was more than just a physical reaction based on some sort of primitive attraction.
He held her in a way that allowed her (and him) to drop out at any moment, but the rest of his body communicated that he clearly did not want that to happen. He was letting her lead them where she wanted to go, careful and deliberate in his movements, as if he could spook her - or lose control. She slid her palm above his heart, testing her hypothesis.
It jumped under her palm.

The next seconds were a pleasant blur. Something snapped inside him as he let go, kissing her deeper and more passionately, scraping at her lips with his teeth. Her eyes closed as his short nails dug half-moons into her scalp, his other hand finding her thigh in an almost bruising grip. She gasped when the deep rumble of a moan vibrated from his rib cage to hers. She pressed closer, first using the hand against his chest to balance herself, then throwing her other arm around his neck.
She let her breath mingle with his as she opened her eyes without parting their mouths. His pupils were wide and bottomless, almost like those of an addict. Their lashes brushed just so.

He was the one who ducked his head away at last. His face was flushed, his chest heaving and fluttering under her hand. Suddenly he stared up at her, and this time she was the one who held her breath. She could not get a read on him.
"I don't want to stop" he said, his words contrasting heavily with his actions as he drew his hand out of her hair. With his back upright, their distance increased to a point where Watson could no longer hold on to him. It seemed to cost him almost physical strength to loosen his grip on her thigh.
'Then don't' she wanted to answer, but could not find the words.
"I don't want to stop" he repeated and ran his fingers through his always ruffled hair. He looked at her as if in pain, all the facial muscles contracted, the big vein on his temple straining visibly against his skin. She could only imagine that her face looked like the polar opposite; utterly relaxed and thoroughly kissed, her skin reddened from his scruffy beard and the heat that pooled low in her belly.

Without another word, he got up from the chair, their knees pressing uncomfortably tight together for a moment before he pushed back, the chair legs scraping against the old hardwood floor. She reached out and managed to brush his upper thigh as he jerked away, making him wince as if she'd given him an electric shock.
"Sherlock!" she called after him, but he did not turn around. He grabbed his coat without slowing and was out of the door before Joan had gotten up. "You're in sweatpants," she chastised the wooden door quietly, as if it would carry her message to him.