Most of what I got from this is:
There's a girl hanging with a group of guys, and she's acting like a girl: Banging one and having some relationship conflict, calling another to gossip about it and thinking too damn hard on what other people mean when they say things about him.
I don't mean this to insult or sound chauvinistic (I'm not that kind of person) but it is my point of view, and I have a hard time candy coating things.
It takes a mature person (2 actually) to be friends, have sex, then keep being friends. You seem to have this down, but he doesn't. You're going to see emotional baggage. You see it already. When you treated him like a "best bro", he probably saw it as a serious relationship move, or passively claiming him as your man. That set him off because he seems to think that this was not in the contract. He may not be aware of your method of classification, or even understand it if you tried to explain it.
Personally, I would have a chat next time you are alone with him and explain that the sex was great, but it's time to let that go because you don't want to ruin a friendship. Keep hanging with him and the others like before and don't worry about catching his eye etc. In fact if he tries to catch your eye you should probably ignore that as well. This is assuming you want to try (key word is "try") to go back to the way things were. If he gives you flack later, tell him to man up and keep the deal, and if he's got any decency, he'll get it and shut up.
That's my take on that situation. Take it or leave it