Gunner had his first KISS :(

OMG! My sweet baby boy isn't so sweet anymore. Today was the day I had worried about for a long time, problem is, I assumed it would take at least another decade to happen. Gunner kissed a girl today...a REAL girl. A non-sister, non-family,
non-family-friend sort of Girl. A girl in his class, girl. A REAL girl. OMG. How terrible.
This is how it happened: I went to his school this afternoon to pick him up. Karlee stayed home sick with her Daddy today, so it was just Gman I was picking up. I walked into his classroom and one of the little girls (We'll call her Smoochie face) came up to me and asked where Karlee was, and if she had to go to the Doctor. As I was explaining to her and the teacher that Karlee was sick, but that she did not have to go to the doctor, Gunner comes up and tells her matter-of-factly, "Karlee is sick. Her tummy hurts." He then proceeded to grab her face with both hands (like her cheeks in his hands) and planted the biggest smacker I have ever witnessed between two two year olds. Like a GOOD six to seven seconds worth. And that's a long time. I'm sure the kiss would have kept on going except that I finally was able to pull my jaw off the classroom floor and remember I was a parent and this was MY child standing in front of me, so I grabbed his arm and pulled him away. The sad thing was, the girl (smoochie face) didn't seem at all shocked about what just happened, which leads me to think that maybe this wasn't the first time it happened.
How (as a parent) do you react to something like this?!?! Ughhh! After I finally pulled myself back together, I explained to Gunner that we don't kiss our friends like that...EVER. My reaction really didn't seem to phase him and I'm not very confident that he will listen.
What on earth am I going to do with this little boy?!?! What if this continues and he gets expelled??? What would I tell my work? "Sorry, but I quit immediately because my son can't keep his lips off of his classmates."
Holy Moly. Let the fun of parenting begin. :)