The April Fools

The April Fools is a 1969 romantic comedy film starring Jack Lemmon and Catherine Deneuve. It was directed by Stuart Rosenberg.

The very evening of the same day Ted promotes Howard, at a house party hosted by Ted, he urges Howard to pick up an available woman at the party and proceeds to show him how. Howard reluctantly tries it on Catherine, who instantly accepts. The two leave the party for the evening and go out for a little adventure on the town, while Ted is oblivious, as he is concentrating on other women at the party.The two find their marriages are loveless as they discover more about each other that night and decide to run away together the next evening. But Ted doesnt realize the other man is Howard until Howard is about to board the plane with Catherine to go to Paris. ........

Source: Wikipedia