The first time they kiss, Harry thinks it has to be by mistake or by accident. They’re together on a nightly patrol of Hogwarts looking for miscreants and rule-breakers, Harry pleasantly tipsy from the drink he shared with Severus to ward off the evening chill of a Scottish castle in winter. Harry had laughed at Severus's suggestion it would be necessary to keep them both warm enough to feel their toes (Ever hear of warming charms, Severus? Are you a wizard or aren't you?) but he accepted it all the same.
As they walk, Harry regales Severus with seemingly endless tales of his own rule-breaking until Severus looks at Harry and laughs, happy and sudden. Harry beams, adrenaline and Firewhisky potent and bubbling through his veins. Severus's mouth is wet and smooth and cool. Harry kisses back with a little gasp of breath that sounds impossibly loud in the night. The angle is off, and it's awkward. They have trouble getting in sync and their noses get in the way. Neither of them is very good at kissing and the Firewhisky isn't helping matters. But Severus is eager and Harry is hard, and Severus presses forward until their legs tangle. Harry's heartbeat speeds up so quickly and, fumbling, he starts to pull Severus against him and then Felicity Abbott-Longbottom from Hufflepuff appears out of nowhere looking for her lost familiar (like father, like daughter) as though summoned by the gods of poor timing and eternal blue balls and the moment is shattered.
The second time they kiss Harry is reasonably sure Severus intended for it to happen. Harry challenges Severus to a game of chess. Harry'd been practicing with Ron and had improved to the point where it took the former-Auror-now-businessman nearly twenty full minutes to put Harry in check, but he assumes Severus would enjoy trouncing him. But Severus demurs and counters with a game of Exploding Snap. Intrigued, Harry eagerly accepts. He's pretty sure he should let Severus win the first game, which is why he'd suggested chess in the first place. However, while he is more than willing to concede defeat in a game of chess, Exploding Snap is his game (Gryffindor Common Room All Time Champion six years running) and it tended to awaken his natural competitive instincts.
But in no time at all he is only just barely holding his own in a series of spirited games. And by the end of the evening they are both exhilarated, faces flushed and energized so that when it comes - the press and slide, the slick and hot – it is still unexpected but not unwelcome. The rasp of stubble against his chin feels electrifying, igniting a heat more blistering than Fiendfyre. It's a slow but inexorable burn, lips alternately rough and chapped one moment, impossibly soft and supple the next. There is a whole journey in the quick swipe of a tongue, the nip of teeth, and the slight suction that eventually follows leaves a circle of red flesh that Harry suspects will quickly turn purple, for Harry has always bruised easily. And when things are about to get interesting, Draco Malfoy's face turns up in the Floo apoplectic with rage over an assertion by some Muggle named Alexander McQueen (who he'll have you know is no Lucius Malfoy!) that Draco's fashion sense is both "lacking and uninspired" and what the devil took Severus so long to answer his Floo anyway, since he was just hanging about with Potter? Harry is suddenly very grateful he remembered to bring along his winter scarf.
And by the third time, well, Harry thinks there isn't any doubt, if there ever really was one at all. This time when Severus's lips brush against Harry's (then brush again), his fingers moving up, framing Harry's face before curving gently around his chin, their eyes close and Severus's mouth settles against Harry's. Severus's lips are wet and Harry can taste him on his own mouth, the striking, familiar scent of the man suddenly made tangible, filling and overwhelming his senses. Severus is everywhere, solid and real, touching him all over, making him gasp and curse - his mouth on Harry's mouth, his wiry body poised over Harry's, his fingers so careful on Harry's jaw. A huffed breath against his lips, softly murmured nonsense against his ear and then Severus is biting at Harry's collarbones, beneath Harry's ribs, then kissing away the sting. He's sliding his fingers through the tangled hair on Harry's belly and around Harry's cock while he rests his bristly cheek on Harry's thigh, and Harry's breath stutters as every thought flies right out of his head and all that's left is sensation. Harry is all coiled energy, trembling all over. Severus is speaking, but whatever he's saying seems like it's coming from a long way off and in a language Harry no longer understands because he's too busy holding on and holding on and coming undone anyway.
When it's finally over it's dark and quiet around them, but Harry can suddenly hear the uneven beat of his own heart. Severus's mouth is so close, and his severe features, which look so stark and forbidding during the day, are muted and almost impossibly beautiful in the dark, though Harry knows he'll never say such a thing out loud and Severus isn't the kind of man to want to hear it. He forces himself not to say or ask anything (for once) and possibly ruin it, but settles uneasily into the arm loosely wrapped around him, resting on his hip.
There is a snort, a mumbled "foolish man," some shifting and the arm around Harry is suddenly joined by its twin and closes tightly around him while a soft kiss is pressed to his forehead. "Have you never heard the phrase 'third time lucky'? I suspect, Mr Potter, you are about to find out that all subsequent times are equally so, errant students and former students notwithstanding." Before Harry can draw breath to reply, there are more kisses - lavish and sweet and full of promise. Harry spares a moment to give thanks for his unbelievable luck and with his own lips returns both the kisses and the promises.