This thread is almost a year old you may or may not get responses all the threads you are answering are old ones.
This behavior in itself seems harmless, in my opinion. The problem is how others react to it. People in this country are so obsessed with sex & keeping kids away from it. The fact that people will suspect sexual abuse & involve Social Services is the biggest problem here. Once they become involved, they can become more then a mere nuisance. So the problem isn't your son's behavior, but peoples' reaction to it. My suggestion is that @ the very least, talk to him about appropriateness of the time & place. If you try to stop him, if he's already doing it, this could cause shame & resentment inside of him, & could cause him to be very rebellious in teenage years by having a lot of unhealthy sex because of a feeling of being repressed. Just my opinion. I hope it helps.
I worked in a daycare for years and I can reassure you that it is normal. Television and even watching mom and dad hug and kiss can be culprits of an early active sexual interest. It can even be that he's just a sexy person(ex. like a scorpio! LOL) Like RockRose said, SUPERVISION is key.
This used to be called "playing doctor". You want to stop it when you see it, and wow the daycare didn't stop this, but now it can be stopped.
Your son and another child his age (6 and 5 are the same age developmentally) got involved in sex play. They need more supervision.
The kissing up the arm and asking the girl to marry him are TOTALLY learned behaviors. Somehow, this boy has been watching old black and white romance movies.
I do think you need to have a SERIOUS talk with him about cutting this sexual behavior out, or he will be in real trouble. But I wouldn't worry that he's a sexual predator - he's a child who acts out on impulses and isn't supervised enough.
Best wishes.