There must be an innocent party otherwise the affair won't work.

Mr A is having an affair with Miss C
He presumes Mrs A does not know
The affair continues in this illicit way.
One day Mrs A says I know you have been having an affair with Miss C, I have known all along, but carry on it does not matter.
Now the affair is no longer illicit, and you know what all the dynamic, all the fun has gone from it
it is like a flat balloon.

We need an innocent other to fuel our illicit behaviour.