It's un-written law, best-mates and girlfriends do not mix (in this context) and it's a line that really shouldn't be crossed if possible. Could you imagine how your boyfriend would feel if you ended up with his best friend. Try imagine you in that situation and realize how ****ed up it is. However you seem confident in the fact that cheating is out of the question. Looking at what you're saying, it's possible it's just a quick crush. If not, you need to decide what's going to happen, as much as I hate to say this, someone will get hurt if things turn for the worse.
The second couple do sound insecure, and it's rather worrying that your boyfriend is interested in just a 24/7 sex relationship, especially theirs! Talk to him about it, express your opinion on the matter, if it's meant to be he'll understand. To be honest it sounds like a young adolescent just wanting sex, which you'll need to come to terms with weather or not you're happy with this.
I wish you the best of luck and hope my dribble of two paragraphs make some sense.