Pre-watching a box set on Netflix by yourself, on the quiet, that you would normally watch as a couple, because she never seems to be in the mood, and you don't have the patience to wait until she is. Later when watching it together, she will be none the wiser.
Gary was bursting to watch season 2 of Narcos, but Sally was too tired, so he engaged in a spot of Serial Adultery.
The modern definition of Adultery sex with someone other than your spouse where you lie to and say you didn't have sex with them.
But if you do have sex with someone else and let your spouse know about it or what people call swinging ,it's not adultery.
He fucked her but it's not adultery because his wife knows about it.