But the spouse is the one who went there.. and chose to stay there.
It's easier to be angrier at the outsider, rather than the one you love.
While in high school, our oldest son gave the security code without our knowledge to his friends to use the house while we were away as a family for two weeks on vacation. I was appalled and felt violated when it was obvious someone had been there. We assumed that they broke in and called the police. My son admitted to giving the code to two kids when asked by the detectives. It blew me away.
I called the kids 'the miscreants'. His friends had unsavory behaviors too. I kept wondering why it was so easy for me to be furious at those kids.. but not my son. I knew the majority of the blame was on him.. and the spouse and I for the situation created. It took me months to even feel a glimmer of anger at my son.
Denial.. protection.. who knows.
But when a deep hurt occurs it's easier to blame the knife rather than the hand that wields it.