I know you are waiting for the expert to weigh in here, but thought I'd offer my take on this situation. I believe I would treat this less about kissing and focus more on keeping one's hands/body parts to oneself along with working on personal space/boundary guidelines.
For whatever reason your son is opting to focus his attention to girls and kissing, but what will serve him best now (during this phase) and in the future will be for him to understand that everyone has personal space and that must be respected.
If at some point, he comes to you saying someone has...whatever: taken his pencil, taken a chip from his lunchbox, tried to eavesdrop, etc...follow up by reminding him that those offenses are personal space issues. He'll begin to understand that he wants for himself what he needs to be giving to class mates.
Finally, for now, regarding the kissing, I would put that on the "house rules" list of no-nos if you think that would help. Just for now, make it a rule that he not kiss girls in his class. When he does, and the teachers should let you know, have a consequence in mind and then implement it. He could possibly write a note (he can dictate and you can write it, he can sign it) offering an apology, stating he hopes to make it up by being a responsible classmate. Something along these lines: Katie, I am glad we are friends and I am sorry for kissing you at school today. I will not do that again. Your friend, Zach.
When he no longer exhibits this behavior, take it off the rules list. He can have that opportunity to feel proud that he has successfully made a habit of not bothering classmates in this way.
Now that he has been told to stop the behavior, firmer limit setting and discipline is in order. He should be told that any further episodes will result in an immediate time out. Should he repeat the behavior, a time out of ten minutes would be suitable and underscore the serious intent of the direction. While his intentions may be fine, the behavior is not.
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