Sometimes its understandable, even if you dont condone it, that goes for more than just cheating. I dont know that anyone should EVER test your loyalty by confiding in you or involving you or using you for cover.
I have one very horrible experience in mind in which I was used for cover and someone (aswell as their bloudy family) got me very, very drunk, I was a punk kid too, and someone I was out with, misguidedly believing they were friendly and wanted to involve me in things, instead he was spending the night with someone who was pretending an interest in me as her cover, while I was passed out drunk in the guest room of his fiancee's house. Guy was engaged. I was at his wedding reception. So was the mistress. He was buying her drinks and sweet talking her while between speeches and dancing with his newly wed.
The other thing about this is that if I've known someone, been a friend of there's longer and never met or dont have a relationship with their spouse or fiancee or partner (not the scenario I just described though, I was an associate, acquaintence or workie of that user at the most) then its only logical I'll owe them a debt I dont owe their SO, although it doesnt mean I'm fine with the cheating, especially not if it turns into a real pattern of deviousness and I'm implicated or something.