Whilst I can understand the argument of C not being at fault, I can't say it holds up to scrutiny. Your actions always have consequences, not just on the cheated party, but indeed sometimes on the cheater themselves.
There was a girl I knew at University who cheated fairly early on, got found out, lost (at the risk of sounding odd) a lovely boyfriend, and went on to have a string of flings.
When she did get into a relationship, she was extremely suspicious and it ended abruptly, her actions had opened up a world of what people are capable off.
Anecdotal I know, but I feel comfortable just saying if you want to sleep with someone, don't do it in a relationship. Get out of the relationship and do what you want...