If one of my friends knew that I was being cheated on and didn't tell me? When I found out I'd be out a partner and a friend. Some secrets aren't meant to be kept. There's a fine line between protecting someone and hurting them in a situation like this. Give me the information and trust that I'm enough of an adult to handle myself.
This is exactly how I feel. I would not be upset or hurt if a random acquaintance didn't tell me but a friend? If a friend knew something like that and didn't tell me, he or she would no longer be a friend. What are they risking? A difficult conversation? Awkwardness? Being wrong maybe? At the very worst they are risking being on the receiving end of "shooting the messenger" syndrome. Which I understand is unfair and not fun but still not even in the same uiverse as me risking my life through potentially getting an STD because I didn't know. Or being humiliated when I finally find out and find out that all my friends knew. To me, a TRUE friend will stick their neck out and tell. I know that I would do it, no questions asked. If I lost a friendship over it, I would still know in my heart that I did the right thing for the right motivations, to protect my friend and nothing more.
When my DH found out that his ex-wife was cheating, he called the guy up and said "either you tell your wife or I will. She deserves to know". I respect him immensely for that. I think it was absolutely the right thing for him to do. The other guy did end up confessing to his wife, and they decided to stay together and work it out, but at least she was able to make that informed decision and their marriage got the counseling that it needed. Not every affair is a permanant relationship
killer, but I believe that everyone deserves the information with which to make INFORMED decisions about their relationships
Now the OP's situation is different, in that she is not friends with the wronged parties, only the cheaters. I think in that case, I would talk to the cheaters and tell them that they owe it to their partners to come clean. If they didn't, I personally would not be able to attend the wedding or be friends with either one of these people. I do have to agree that in most cases an acquaintance shouldn't be the person giving the news. iPersonally, I'd still want to know, even if it was only an acquaintance who told me, but I don't think an acquaintance has as much or any obligation as a friend to tell.