[PSY]

Maybe it's not typical for G's friends to sleep with someone while they're all hanging out at your best friend's cabin? I don't know what "hanging out" generally consists of, but if it DOESN'T usually involve sex, then G could be upset for a number of reasons. He could, as Chris suggested, be jealous of what his friends are getting. He could be concerned that you'll emotionally hurt his friends in some way (which would leave him to pick up the pieces). He could also be angry about your "stealing" his friends away, when they should have all been hanging out together. Who knows?

The real question here is whether or not you want to spend time around your "best friend" and G. Personally, it doesn't sound like you have many reasons to keep hanging out with them (aside from the occasional laugh, and perhaps the fear that you won't have anyone else to hang out with if they're gone). Ask yourself whether you can be happy with these two in your life. If the answer is "no," then it's time to ditch them.