Brown mediocre? A ridiculous statement. I admit that I didn't much love the last revival of THE LAST FIVE YEARS, but the film is surprisingly good, and perhaps that has something to do with the casting of Jeremy Jordan and Anna Kendrick. Hard to imagine a two-character musical film working so well, but it does. And while BRIDGES did not have the success it deserved on Broadway, the score was remarkable, and Brown won the Tony for it.
As to your original question, I don't think either of them is to blame. A relationship goes sour, one is the first to be unfaithful outside, and then it is over. Nothing unique about that, although it is sad, and it is Cathy we hear from, having a tough time moving on. Gay or straight, most of us have been there, and yeah, it hurts like hell.