Social conditioning, once again. We’re French, and it’s totally possible for us.
Yesterday, among strangers, at the local lake, my 42 year old friend was topless, playing with her 7 year old son and her 5 year old (naked) step-daughter. A 36 yo pregnant friend was wearing a string bikini around her husband, 6 yo daughter, and in-laws. It was a non-issue.
Not saying all rules can always be broken, but they’re heavily context- and culture-dependant.

My partner of 15 years has a girlfriend of 25 years. She’s been married for 30 years. We’re happy.