Well, only GuyA can really answer that question. Who knows why he did that. I'm sure he thought he had reason, or maybe he was just mad. What you're seeing here though is a classic example of what happens when people don't communicate.
Really, I don't want to sound mean, but if GuyB was really GuyA's best friend, how in the world did you ever expect to keep all that secret? It just doesn't work like that. Being a guy (and trying not to sound stereotypical), I know that guys can be very gossipy.
You might want to reevaluate your position in all this. You're friends with GuyA, but didn't seem to have any qualms about sleeping with his best friend, an activity that was obviously going to hurt his feelings (or else you wouldn't have kept it secret).
Just some things to think about.
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