This was a 1970's era soft porn movie about a couple who partake in sex with others in a very open relationship. They end up betting each other to see who can seduce another couple. The catch is the wife seduces the woman and the husband seduces the man. The movie is set in Europe I believe. The movie was made somewhere between 1973-75.
There was Bob and Carol, Ted and Alice, the US movie with two couples all in bed together. From 1969 it featured a score by Quincy Jones, R.I.P. There's also a series on now with Jane Fonda and Martin Sheen where the two husbands end up together and the two wives end up business partners, I think. There could be a foreign film like that, there has to be, but I don't recall one.