Is AUTHOR wrong in the post ? POST : I don't want to give too much away. Throwaway account. Going to just speed it up. 

Very good friends with 2 people. Played alot online etc. 

2 very good friends, found out friend A was cheating on his girlfriend/fiancee, I stopped talking to him, I thought it would be best for me to leave the situation, and just not get involved. I ended the relationship with him. Told him I didn't appreciate what he was doing, and told him to rethink his life. I didn't want to ruin his life, and told him to spend more time with his wife(newly wed) and kids. 
About 9 months later, 
Now Friend B is starting a relationship with this bit on the side from friend A. And I just can't be fucking arsed now.
I just don't appreciate the whole situation from what's been going on. Friend B was the one who told me about the fling, and they were even closer friends than I was with either of them. They still talk also. Don't understand why he's going out with someone who would happily sleep with a practically married man (they are in fact married now) 

Just feel like because of the whole situation, I've lost 2 good friends. 
I understand he might be happy with her. I just can't justify it morally.  Or am I just over thinking too much? 


Edit: I know I'm an asshole for not telling the partner about the cheating. That's not what I'm asking. There are kids involved, and as much as I want to see him hurt, I don't want to see a family argument with kids involved because of me.
A few people know about the fling, including a family member and hasn't said anything, I don't think it's down to me. 