mstang, you said, "Honestly, I think he will (embarassed or not) laugh and say "that's what guys do" when I bring it up. He may surprise me and not do it, but that's my guess."
The thing is, you should step up and say what needs to be said. Do it with humour as well, don't just go in guns blazing expressing disgust.
He COULD be doing this deliberately to be confronting, or he could just be simply a thoughtless kid. It really makes no difference, because YOU can horrify him, far more than he can hope to horrify you.
You have so much more ammunition than you realise.
1) You are an adult.
2) You are a parent figure.
3) You are female.
All of these can be used to thoroughly embarrass him. Do not fear for his fragile little ego, or worry you are going to give him hang-ups - frankly, he NEEDS a few more hang-ups! So go for broke.
First, if you go over to shut the door, do make sure you talk to him as well, making eye contact, BEFORE you shut the door. Say something like, "If you absolutely MUST 'relieve your inner tension', then do it discreetly. If you were sharing an apartment with someone they would give you heaps for being so obvious."
If THAT doesn't embarrass him enough, then the next time you say, "In this house, we perform sexual acts, solo or as a couple, in private. Mind you, if you would like that to change then I will let your father know that it is OK by you if we leave our door open from now on when we're having sex. And I'll not worry any more about keeping my voice down."
Each time, you up the ante, making it clear that:
1) You and your husband are sexually active and enjoy a healthy, fulfilling sex life.
2) This is YOUR house, YOU set the rules. Respect must be shown to all housemates.
3) Privacy is best, regardless of where you live.
"Shaking hands with the unemployed" is best done in private because it's an important habit to get into. I posted a few months ago about my oddball cleaning man, who difficult child 1 witnessed being arrested on a city train - he had been "spanking the monkey" while riding the train home, thinking it was an empty train carriage and totally not seeing the signs warning of closed circuit television. At least, I HOPE it was an empty train... my cleaner is definitely difficult child, bipolar.
Whatever you do, do not show disgust, do not show horror. The most effective way to shut this down is to use humour. Collect things you can say to him.
"That reminds me; I must buy more toothpicks."
"Put it away, little boy, it's too small to be out."
"Put it away before it catches cold."
"I can give you some depilatory cream - for the hairs on the palms of your hands."
"Doing that in public just screams at all observers that you're unable to get a girlfriend."
"Keep THAT up and you'll get RSI."
"Don't expect ME to kiss it better when you've rubbed all the skin off."
I know some of these are cruel, but if he is doing what he is doing to horrify, shock and be cruel, he has to learn that he cannot win this argument.
Once your kids realise that you not only can't be shocked, but you can turn the tables back on them to a horrifying extent, they will back away quietly as from a tiger about to charge. I got husband on side with all this; he became more demonstrative in front of the kids (not to any obscene extent; we do have SOME standards) just enough to have the kids under control.
A positive side effect - the kids knew we wouldn't get red-faced and mumble if they had questions about sex, so they were more willing to come and ask US instead of their ignorant friends. They still hold back a lot, but when questions MUST be asked, we get asked. The girls ask me to go in with them to the doctor, for things like pap smears or similar.
And I will try to express this as discreetly as I can, keepingsite censors in mind. Use this on your son too, if you can bring yourslef to.
difficult child 1's friends were shopping at the mall with us (a few years ago - they were still in high school). While I was busy with a shopkeeper, one of them told difficult child 1 a really rude joke. difficult child 1 dared the boy to tell me the same joke.
So the boy, thinking to shock me (silly boy!) did. The joke was, "Why can't you ***[what your son is doing under the bedclothes] with these two fingers?" (he holds up two fingers).
The answer - "Because they're mine."
I just looked blankly at the boy (mind furiously working) and finally said, expressionless, "Why would that stop me?"
The effect was almost magical. Since then they've never even used a swear word in my presence.
I'm also intrigued by the "whatEVEERR" response - we see it here only in US TV and movies. We don't hear it. We might say, "whatever," but it has a totally different meaning (and is not disrespectful). For us, it means what it says - "whatever you prefer. Seriously. You choose." As in - "we could go have coffee, or tea - whatever."
Sometimes an Aussie kid who's been watching too much US TV will try this on and be met with total uncomprehension, and a request for clarification. "Whatever what, darling? You want me to choose from options you are giving me? I didn't hear any options, so please explain."
This doesn't mean our kids aren't disrespectful sometimes. Just that it is expressed differently here.
Marg