For me it is knowing she is getting ready to go and get fucked. The tension, torment, and humiliation is nearly unbearable. Second is when she just goes and has her time then comes back and tells me or brings him home and banishes me to the guest room.
My wife does not want me to be in the same room. She thinks it takes away her pleasure. I am actually happy about that. Hearing is bad enough seeing I don't know if I could handle.