Well a little kiss between friends sounds harmless sounds like the Teacher made a deal of it,has he got a sibling or relative who wanders around Naked, your 5 year old may be emulating, has he seen any grown up movies on TV you are watching and hes around ,most regular movies now are full of it,we disreagard it but it intrigues young minds.Hey you said you had a talk , Did you ask him Why he did it???
I think you need to reinforce to him what is appropriate and not appropriate behavior. I am constantly amazed how children pick up on things in their environment that we don't even see-- people kissing, hearing about private parts, and stuff like that. Even if you know that your son has not been exposed to this stuff, if he saw anything like this ANYWHERE or heard it ANYWHERE, even for a second, it will spark his curiosity-- he's 5.
Spend some time explaining to him why some behavior is appropriate and why some is not. If the behavior continues, you should try some incentive behavior modification -- rewards for positive behavior, and then time out for misbehavior.
Overall, this is nothing to really worry about.