relationships are very personal things, as long as everyone is rigourously honest and open about their boundaries and feelings and motives etc, and there is equality of opportunity and respect for all partners, then I see no reason why a polygamous or polyandrous marriage can't work with the same degree of success that on average monogamous ones do.
could I do it? not sure, probably not, doesn't mean others can't make it work, and its really none of my business what makes other people happy as long as it's between consenting adults.
FWIW, I'm not sure the french do have a different attitude about infidelity in their own personal relationships, I think they have a different attitude when it comes to people in the public eye, they don't think that a rich man having a mistress makes his business/political decisions less sound.