There fist kiss was well over one hundred years ago, Mayuri was still the third seat of and Zaraki was just starting out as a captain.
At the time It was a total accident. Zaraki had been seeing a blue haired woman with a pale complexion, It had been late and Zaraki had drunk more then his fair share. He would not have been able to sneak up behind the third seat if Mayuri had not been consumed by the report in his hands. The younger man would have noticed the hands wrapping around his waist if it hadn't been for the breath in his ear. The genius that was Mayuri Kurosuchi would have seen it coming if he hadn't spun around into those arms and unknowingly into that kiss.
Kenpachi should Have noticed that no those were not funny earings. Had he been a little less sober or a little less dark he would have realized that the woman he was seeing had much longer hair. Had Kenpachi hadn't closed his eyes when his lips covered the others he would have seen a hard golden stare instead of soft hazel eyes.
He did notice however when his 'woman' Pushed him away and made a few angry sputtering noises before being able to form words. "What do you think your Doing?!" Mayuri had backed away and covered his mouth with his hand, Eyes trailing up and down his molester before he added a snide "...captain."
Zaraki just shrugged and stating that "So I guess your not my woman." Before the funny looking angry man could yell at him anymore Zaraki headed down the path, Intent of finding his woman, and completely making sure that she was indeed his woman before going any farther then well he didn't know yet. Had Zaraki bothered to turn he would have seen a slightly confused but mainly pissed off man glaring holes in his back.
"Honestly, Captain Urahara should have let me alter myself into having lazor eyes, Then it would at least be interesting dealing with these idiots from Eleventh." Mayuri glared one last time into the direction Zaraki had taken off before heading somewhere 'safe' to finish reading through these reports.
After there incident It was never discussed, but mutually agreed never to bring that night up with one another. Though It should be noted that Kenpachi ended up ending it with that woman that night because she just wasn't as good a kisser as the person he couldn't remember, And Mayuri went out of his way to make sure that the Eleventh Division got less then satisfactory goods, at least until he was told to stop, several times.
They Went on with this uneasy trues for nearly 50 years before anything else happened between them.