Liberal saunas are much more practical than swing clubs: being naked in a jacuzzi already takes you halfway past the bureaucracy of having sex with strangers.
Paris is home to some of the best swing clubs in the world, which they call clubs libertins. They are basically nightclubs with benefits (with rooms where you can have sex). These swing clubs are not at all an underground scene: they are very fancy places, with high-level services and facilities.
Those who think swinger clubs are synonymous with just underground nightclubs, don’t know the Les Chandelles in Paris, an awesome club échangiste or club libertin as they are known in France.