I brought up things in my post to support my point of view not because I was arguing with anything you or anyone else had said so responding with stuff such as "arguing against yourself" isn't really helpful.
You explicitly stated that my argument was based on assuming one of them and brought another up explicitly as a counter-argument to my posts. You did not bring them up in isolation.
At the end of the day its not the OP's marriage so I don't see why he should feel any kind of guilt.
For the same reason that someone knowingly receiving stolen goods should feel some guilt despite the fact that they didn't steal them. If a person takes part in something, they are responsible for doing so.
That's down to the wife since it's her relationship.
That would be true if no-one else was involved, but obviously that isn't the case. He wouldn't be responsible for the wife's actions, but he is responsible for his own.
All IMHO anyway, if you feel differently that's your choice.
Choice is the foundation of it all, including the choice of actively helping someone else do something wrong.