I was in my 20s the first time that a couple I knew fooled around with open marriage. There have been others, because stupidity has reruns. Mark I'd known from high school. Debbie was two years younger and very pretty.
A bunch of us had gone to a concert at Long Beach State but Debbie was missing. I asked where she was... "Oh, she's on a date with her psychology professor".
After that sank in a bit I asked "well... what about jealousy n' stuff?"
My friends were amused at my Medieval ideas about love and marriage and assured me that my dire predictions of disaster were ridiculous.
So Mark allowed his pretty young wife to bang her psychology prof for the rest of the semester without any objection. And then when summer came Mark arranged a week long camping trip with an old flame of his from high school.
If you guessed that this ignited a fury of jealousy in his faithless wife you win a prize. The marriage blew up along with Debbie's temper. Apparently their emotions were in fact stuck in the same Medieval world that I inhabited and hadn't made it to the 1970s. Go figure. At least no one got knifed or shot.
There is an old (85?) year old couple that takes their walks together twice a day.
Another elderly neighbor said the other couple used to be swingers back in the day and have wild parties at their house! Really weird (and gross imho) to think about. Yet - here they are still married and devoted to each other!?
One of my friends told me his wife said to him “I am no longer interested in sex”. When she caught him running around, he retorted “I thought you weren’t interested in sex”