The first time John and Sherlock kiss, it's an accident. Sherlock's just found himself a wonderful case, and, in the joy of deduction, he kisses John squarely on the lips and turns away, talking almost the whole time. It takes both of them a second to realize what's happened, and neither of them mention it to each other again. However, John lies awake in his room for many sleepless nights after, wondering why Sherlock had kissed him. He also wonders why he liked it. After all, he's not gay.
Not at all.
Is he?
The second time John and Sherlock kiss, it's an experiment, or so Sherlock says. Sherlock calls John in from the next room and asks, astonishingly nonchalantly, if he can experiment with something – for a case, of course. When John agrees, Sherlock walks over and, slightly hesitant, he takes John's face in his hands, then kisses him very deeply.
Very deeply indeed.
John's blush lasts for days.
The third time John and Sherlock kiss, John's drunk. Really, truly, stupid drunk. His latest girlfriend's just broken up with him, and he really liked this one. Sherlock's not drunk, but when their experiment from a while ago came up and John asked if he was gay, Sherlock neither denied nor confirmed it. However, when John asked him if he wanted to "experiment" again, he simply went over to John and…well, experimented.
And experimented again.
And then experimented some more.
The fourth time John and Sherlock kiss, it's romantic. John's hands are on Sherlock's hips, and Sherlock's hands are cupping John's face. Sherlock's just come back from a very dangerous case, and John's been worried sick. As soon as he stepped back into the apartment, John rushed towards him and hugged him, then kissed him.
Then slowly began to move them into his bedroom.
And then…well, that part was quite romantic, too.
The five hundred and fourteenth time John and Sherlock kissed, it was on their wedding. They were grinning so widely during the kiss, it felt like their faces were going to split open. It went on a little longer than strictly necessary, and Harry wolf-whistled quite loudly, making them break apart and John smile embarrassedly. Sherlock just grinned and kissed him again, and everyone clapped.
There were a lot of kisses after that.
A lot of kisses.
A/N: I used to write a lot of Sherlock fanfiction, and I think this is definitely how John and Sherlock would get together (if they ever did the do). I'm not even an active Johnlock shipper anymore - I left that fandom quite a while ago - but this remains my headcanon.