Be smashed. I once stumbled into the kitchen hungover to be told by my house mate 'I think I ****ed [our other housemate] last night. Did I get with her at Cindy's?'. I laughed so hard I was nearly sick, though to be fair that didn't take much that morning.
I think it depends on the kind of person that you are. For example I know a woman who was with this guy for 10 years (and still are a couple). Only he's been having an affair with his sister-in-law for the last 4 years. Clearly it's not a problem for them š. Also, the woman in question is aware but chooses to pretend it's not happening for the sake of the children...