What was the name of an old cinemax after dark special movie, the plot was a couple was on vacation on some beach and then the be-friend this other couple who they then end up hooking up with the other persons partner in secretcy without their husband or wife finding out till later.

There was a movie called, "Linda".
With Virginia Marsden and Richard Thomas. Sound like it could be the one you're looking for.

A Muderous Affair?