"Here’s where it gets like high school: Nash was staying at Stills’ house, and gave Coolidge Stills’ number to call so they could make arrangements. But it was Stills who answered the call and told Coolidge, gosh no, Graham can’t take you after all, but I will. Coolidge, who wasn’t thinking about romance at all, accepted Stills’ invitation — and got a frosty reception from Nash backstage at the show.
Coolidge and Stills became romantically involved shortly thereafter, but when Nash told her about Stills’ deception, she decided that she preferred Nash to Stills. Coolidge insisted they tell Stills personally, so they drove to his house to break the news to him — and it went about as well as you’d expect. Browne writes: “After the poolside conversation, Stills refused to speak to Nash, and the increasingly delicate thread that held them together finally snapped.” It wasn’t the main reason CSNY split up, but at that point, it didn’t take much to make things worse.