In the first case, they were boyfriend and girlfriend, and he cheated on her with a mutual friend. I couldn't wrap my head around that - cheating seems like the stupidest thing in the world to me. I don't think he wanted to keep his old girlfriend and new girlfriend at the same time. He just got in a situation where he fell in love with someone else and didn't have the guts to break it off with his girlfriend first. I was pissed at him for a couple of years, but as time went by I got over it. He married the new girlfriend, they're doing okay and have a kid. The old girlfriend moved on and married as well (also with a kid). And if learned anything from Hollywood it's "All's well that ends well." When I lost touch with him years ago, life got real busy with careers, and it's only now that we are back in touch.
The other situation was a married couple. Two married couples, in fact. That right there sets it on a whole other level. There are oaths, promises unto death, that were discarded. And as far as I could see, the cheaters were content to keep on having this relationship indefinitely, betraying their spouses till death do they part. Both of their spouses suspected, and eventually they were discovered incontrovertibly. Cue screaming, bad blood, and lawyers.
Now, if they had fallen in love, gotten divorces from their spouses, and remarried, I would have been weirded out by it but I would have learned to live with it. At most I would have felt awkward around them. But the way things panned out, it literally shattered our group of friends. Even now, some ten years later, I still only see people from the other side at accidental meetings or funerals.
Even that doesn't get to the heart of why I just can't get past it; why I still can't entirely forgive them.
The fact is I saw what was happening. We all saw what was happening. And we didn't stop it. I clearly recall a moment when I felt like screaming at the two to stop messing around and do the right thing, and the moment passed. It is my most shameful memory. I can probably come up with some reasons why I didn't actively intervene, some reasonable excuses, but it doesn't change the fact that I failed. I failed in how I think a friend should act. I participated in that betrayal. I can't forgive myself for that, how can I forgive them?