The April Fools (1969)

"He has a wife. She has a husband. With so much in common they just have to fall in love."

Newly-promoted if none too happily married Howard Brubaker leaves a rowdy company party early with the stunning Catherine, whom it turns out is herself unhappily married — to the boss. They spend an innocent night in New York becoming more and more attracted to each other, so that when Catherine announces she intends to leave her husband and return to Paris, Howard asks to go along too.