Why do people keep saying that Terry slept with his best mate's girlfriend?
He didn't. He slept with his best mate's ex-girlfriend. Doesn't excuse him for his behaviour, but it's not the same thing.
As for the story, even if it's true it doesn't exactly shed Terry in a good light. What he did had the potential to cause unrest in the camp because Bridge was in the squad and the players know him etc. That said, I find most of the story is highly unlikely for reasons already mentioned. The players having a rift where Beckham intervened would not surprise me at all, knowing what our players are like...