La Moglie di mio padre (1976)

A wealthy man suffers from impotence whenever he tries to make love to his wife. Since Viagara hadn't been invented yet, his swinging liberal doctor instead recommends a strong dose of infidelity. Before long he is rolling in the hay with a young lovely and needless to say his impotence problem is cured (it's amazing that he doesn't have a heart attack though). His wife is a little bitter, however, and decides to embark on her own affair. And if you're wondering with whom, well, just look at the title.