I know the French thing's a stereotype and nothing's that simple. But, well, I've lived in France and, ime, there is a socially validated option that we don't have in the UK. And it has boundaries that everyone knows, designed to rock boats as little as possible.
It's basically don't ask, don't tell, don't be a twat. You don't get suspicious when your husband/wife goes out to book club or the gym, wearing a nice outfit and perfume, and comes home late. You make an obvious and educated guess. You say nothing about it. You don't see your 'little friend' so often that it prevents either them or you engaging in your marriage. You don't miss family events for them. You don't mope about the pool on your family holiday, you keep extra-marital messaging to a decent minimum.
Not everyone could live like this. I could, actually. Unfortunately, both my husbands were outraged by the very idea, so I bought into the exclusive monogamy they said they wanted. Both cheated prolifically. All the bloody lies! The coercing friends to lie for them, the total life damage that causes. It's impossible to believe they weren't getting a kick out of deceiving me, letting me down, betraying me. Because I'd told them I could live with non-monogamy, just not with being disregarded. So they did both.
Incidentally, XH2 cheated on his current wife with me while they were engaged. He had absolutely no reason to - it must be part of his inbuilt drive to be a dishonest arsehole, proving some kind of point to no-one in particular.