My parents would KILL me if I did anything like this. Whereas my parents-in-law are standing firmly behind WH. At one point, my F-i-L said, "I have to support my son." I said in response, "You can support and love someone and tell them what they're doing is wrong." He just looked at me and said, "I am not going to do that."
On a separate occasion, I expressed my feeling of betrayal to my M-i-L; told her I felt let down by her response to WH's affair. I told her I felt she was just waiting to meet my replacement. Her exact response was, "What else would you have me do?"
Both of my parents-in-law have had affairs. I don't know whether this is a factor in their behaviour, but am guessing it contributes.
It must be that because we know our families would kill us that our moral code is different to our WS's codes - obviously their families don't seem to be overly bothered.
My parents-in-law continually say that WH wouldn't have cheated if he weren't unhappy. My response is always, "Well, I was unhappy too, but I never cheated." They don't accept this. They always just keep quiet to this and then change the subject.
Out of interest, do either of your WS come from conflict avoidant homes? Mine is supremely conflict avoidant.