NC for this as I've been posting a lot recently under my usual name and don't want to out myself.
I'm worried about a situation with exp and ds: essentially what it says in my thread title.
DS has a sleepover once a week with my ex. Ex is short on space at the moment and has moved DS's bed into his bedroom. Fine for just the two of them, but DS says his dad's GF is sleeping in there as well (DS in a single bed, ex and GF in the double. It's not a huge room either.) Obviously I don't know if they're having sex in the same room and it's not something I can ask. But I'm v.concerned nonetheless. Ex has a problem with boundaries shall we say. He lets ds watch films and play computer games I don't approve of, doesn't curb his language around him, tells him what I think are inappropriate stories/discusses adult issues with him without 'rewording' them in a way (I would think would be) appropriate for a child.
DS is 9.
My ex was EA to quite a massive extent, I posted a big list of stuff about him on the red flags thread and even I was shocked to read it back 
We've got a difficult relationship, I don't like him and he makes me nervous. We're civil atm but I like to keep my distance.
We had an incident recently with DS and a family member of similar age who came for a sleepover at mine. I put them in a double bed and on hearing voices I went to listen at the door to find that DS was encouraging the other child to take his clothes off and for them to rub their penises together. I was fuming, separated them for the night and told the other child's mum what had happened the next day. When I'd calmed down I had a sit-down talk with DS about sexual feelings being natural but something for adults, that private parts are private, that he shouldn't be trying to touch anyone else or asking someone to touch him, etc.
Ex said at the time it's just what boys do and I was overreacting.
I thought DS had taken my point but he went into school that week and made sexual comments to a girl in his class and I was spoken to by the teacher. I reiterated my talk with DS and said that sexual behaviour and comments to others were inappropriate, again that they were private, adult matters.
Nothing's happened since with DS but due to the above and DS saying that his dad and GF are sleeping together in the same room, I'm worried again and not sure what I should be doing. My main concern given ex's lack of boundaries is that he and gf are having sex in the same room or at least sexual contact and that DS might be (or become) aware of this. Whereas I'd say it was obviously highly inappropriate behaviour to most people I'm not sure ex falls into the bracket of 'most people'. (Nothing personal against his gf but she is a lot younger than him and not too bright herself, from what I know of her.)
Ex is antagonistic by default and won't take kindly to me raising the issue, there's literally no easy way I can broach it. I'm thinking of saying that DS can't sleep over unless/until he gets his own room back. What I want to say is that he (ex) has six days in the week to sleep with his girlfriend (they don't live together) and I don't have a clue why he can't keep one night special for just him and his son to have their quality time. His reply to that will be that I'm "controlling", trying to stop him seeing his son etc, all stuff I've heard a million times before.
Do you think I'm overreacting/worrying about nothing, and if not, what should I do?