Corps brûlants (1976)

A teenage boy named Eric already has a gorgeous sweet-natured brunette girlfriend of his own age-group, yet he develops a lustful crush on a considerably-older flaxen-haired women named Helene when he sees her and her male lover Francois playing tag on the beach and then having wild intercourse on the dune-grass. An intimacy soon develops between Eric and the comely matron; as Helene sobbingly explains to Eric, she is feeling extremely lonely and "sexually neglected" due to her live-in lover Francois's being away for a few days, and so the teenage boy is perfect company in her time of need. During their time together, the boy naively assumes that the lady has genuinely resigned her heart to him; however, when Francois returns and Eric catches them making love once more, he realizes to his shock that she had merely been using him as a "boy toy" for temporary comfort. Now heartbroken, Eric must make sense of his feelings; he soon realizes that his relationship with his own similar-aged girlfriend is far more suitable and satisfying for him, anyway.