My son and his friend were having sex?

Hi my name's Luke, I'm a gay and a single parent bringing up my 6 year old son who's now started year 1 of primary school.

The other day my son asked if his friend could sleep over I won't say their names but his friend is the son of two of my closest friends since secondary school and so we talked about it and any way his friend ended up staying over for the night with me and my son.

Later that day I ran the bath for them (my friends asked me to bath him) and once I had washed their hair and that they wanted to stay and play in the bath for a bit with the boats so I told them they could play in the bath for another 20 minutes then I would get them dried and dressed.

So after 15 minutes I went upstairs to get their clothes ready and when I went into the bathroom they were playing with each others penis now I'm a laid back parent and a nurse and worked a lot with young children and know that they explore each others bodies and things so I just told them that they shouldn't be touching each others private areas because they're private for a reason without raising my voice and they stopped.

I got the kids dry and dressed, let them play for another couple of hours then I put them to bed and let them watch a film until they fell asleep and later that night when I was getting ready to get in bed I went to turn the TV and that off and when I went in his friend had got out of his bed and into bed with my son which again had no problems with and I was just going to leave them but when I went to turn around their clothes were on the floor so I just woke them up slightly to put their clothes back on and both of them were completely naked and was sleeping with each other so I put their clothes back on and put them back in bed I let them sleep in the same bed since they were tired and half asleep and left them to get back to sleep.

Now the next morning when I was downstairs I heard some noise and knew that they were awake so I went upstairs to ask if they wanted anything to eat and when I entered the bedroom I saw my son naked on his hands and knees and his friend naked behind him sticking his penis into my sons backside!!!

I was really shocked to see this as I think any other parent would be I know that kids explore each others bodies like I said and that but I wasn't expecting this and this also made me raise my voice not in anger but shock and I made them stop what they were doing and got them dressed and told them they had to play downstairs where I could see them and they went downstairs and acted as if nothing had happened.

When his friend had gone home I sat my son down and asked him what he was doing earlier and he said that they both thought of a game to play and it involves them both being naked and playing with each others bodies and they call it the "Willys and Bums" game and that they play it a lot when their alone so this means that they've done it before and that they want to keep playing it.

So I talked to my friends (the other boys parents) and they said that they had seen them playing it when my son went over to theirs for tea but didn't know how to tell me and just to say my friends are a gay couple so all three of us talked about it and that we don't mind them becoming gay or even being boyfriends and our kids understand that but we just don't want them to be doing things like this at such a young age and every time we catch them we tell them to stop and we try to always keep an eye on them we have also told them to never do it with other kids cause the last thing we want is our kids being bullied or parents and other people thinking things about us.

So does anyone think this is normal? What could we do to stop them from doing anything like this? until their older and know about privacy and does anyone have any idea of how they thought of doing something like this? Any response will be great but if it's anything that's just to offend me my friends and our kids then don't bother answering but anyone suggestions on what to do will be helpful