I think the "germ thing" will backfire on you because they see everyone else kissing and it's really not a great thing to suggest that people are infecting each other every day. I think you have to keep focused on the real issue for both you and the other mother, and not derail them by shifting the reason around. You can explain it in your own terms but it helps if you and the other mother are in agreement and feel you can use the same words.
I think, at this age, it's about affection for each other, and they see it as a natural expression of their caring. I think you can say that lip-kissing is for parents/kids and for grown-ups with each other (or older teens, whatever you want to say). Cheek kissing is for special friends and relatives (Grandma, Aunt Susie, whatever), hugs are great IF both people want to but they can't go around grabbing their friends at school. It's probably best if you really emphasize that not everyone is comfortable with kissing or even a lot of hugging, and they need to be careful in their choices. Also, they need to know that many people don't like to see a lot of hugging and kissing, and they may be teased by others. It lets them know what is socially acceptable.
They may sneak off and kiss anyway - many of us did. It's not the end of the world. I think you are worried about greater levels of exploration. My guess is, the novelty will wear off soon. But you don't want to make it such a private thing that they hide it from you.