The Bliss of Mrs. Blossom 1968

Harriet Blossom is married to Robert Blossom, a businessman who'd rather spend the night at his bra factory than at home with her. One day, Harriet's sewing machine breaks, so Robert sends a repairman, Ambrose, to fix it. It's lust at first sight for Harriet, who convinces Ambrose to hide out in the attic for a tryst. When her new beau shows no desire to leave, the pair begin a years-long love affair right under Robert's nose.