We've gone through this with our oldest two, though they were the ones being kissed rather than the other way around. I'm sorry your babysitter was scandalized by this. Unless kids have been abused or misled in some way, what they are doing is displaying a normal curiosity, and it's completely innocent. But so is nose picking, and that's innapropriate, too, so the question is how to curb the behavior. When our children were talking about being boyfriend/girlfriend, it was always with a good friend from school. Neither of them were comfortable with it, and they certainly didn't appreciate the kissing. We talked about how it's so nice to have friends that are boys and friends that are girls, but they are not ready to have boyfriends and girlfriends. We also explained that it's so good to love our friends but that there's a big difference between friend love and mommy/daddy love, and that boyfriends and girlfriends are what we have when we're thinking about finding someone to be mommy and daddy with. We told them not to hurt their friends' feelings by saying "eewww grosss!" or getting angry if a friend gave them a kiss, but to just ask that friend to give high fives instead of kisses from now on.
It sounds like your son has a real curiosity about the human body. Perhaps it would be helpful to both of you to simply ask him if he has questions about his body. That opens the topic in a way that lets him know that you don't think he's a bad boy or dirty for being inquisitive, and that you respect his need to understand himself and others around him. Talk about all of the great things our bodies can do like running, jumping, breathing, growing babies, pumping blood, etc. After all, better that he hear from you what all the bells and whistles are for than from a schoolmate or, worse, an adult who may take advantage of his precocious nature. Maybe that would stem the curiosity, or, at least, channel it in a new direction. Who knows? You may actually have a future doctor in the family after all.
Good luck with this.