playing "boys and girls"

It may just be nothing, just normal behaviour, I don't know. About 3 weeks ago my son was playing at his friends house, my son is 4 the other kid is 5-both boys. Well, they were caught in the closet. The other boy had his pants down, my son didn't. My son wouldn't tell me the truth there, but as soon as we got home I asked what they were doing and he said "playing boys and girls". I asked how to play that game and he said you pull your pants up and down. I asked what else and he said nothing else. So I told him naked games are not ok, with kids or with adults and that he is not to play that way anymore and he said ok. Today I had the same boy at my house (they haven't played together since last time) and it got quiet in my sons room so I went to peek and my son had his hands on his waist band like he just pulled his pants up.I asked what he was doing and he said nothing, this time it was the other boy who said my son pulled his pants down. This is the 3rd incident. The very first was at the same boys house. They used to watch a little girl who had some issues and taught them how to make the dolls go up and down on each other, but that was like 3 or 4 months ago. I guess it is a good sign that they aren't touching each other, but I was abused from a very young age so any behaviour like this really makes me worry and probably a little paranoid. They are also teaching good touch and bad touch at his pre-school, so that might be another thing contributing to the situation. We are very careful in front of my son, he doesnt see us do anything, except a peck on the lips or a smack on the butt. I am just curious if anyone else had this issue and how worried I should be and how to stop the behaviour, besides totally not letting these kids play together cuz telling them naked games aren't ok isnt working. thanks for reading.

[margypops]

It may be a good idea if they do play together to supervise them not let them play in their room alone ?