I'm inclined to agree with Struggling re your ds's reactions. It's not up to your ds to make decisions about his sister's sexual activity, in fact it's quite inappropriate. Also, having read all of your posts, I do think that your ds is being rather judgemental. So the bf dresses a bit chavvy, he doesn't go to a gentle middle class school and has what seems a quirky, if inappropriate at times sense of humour.
From what you've described of the bf, it does sound like he may well have something on the spectrum, he sounds a lot like my brother. That being said, some allowances have to be made, he has had a really shitty start and has finally got someone outside of his family who clearly adores him. If he really does have some sort of ASD that is a really overwhelming thing to deal with.
I think the rules you have at your house are perfectly acceptable, and actually I think the bf sounds like a gentleman in the way he is behaving around having sex.
It's certainly not nice thinking about your dd doing this sort of thing. But I think you need to think about this. Would there such an enormous issue about it of your ds was ok with the bf? He really can't be so controlling.
You're doing everything right and you must take that on board. Ultimately they probably won't last/get married etc so I don't think too much emphasis should be made about her choice of bf. It does sound to me like he could be a lot worse, at least he is affectionate, makes her feel special etc.