What Do You Think Of Young Children Kissing Parents On The Mouth?

I was saying goodnight to my two year old, when all of a sudden, he kissed me on the mouth. Obviously there was nothing sexual about it, but it didn't seem like something that I should let him do, so I just said "Mommy doesn't like to be kissed on the mouth, but you can kiss my cheek.".

 What else should the child know?
How can sex education come in here?