If my memory serves me correctly, Prentiss refers to Bachelor In Paradise. In a separate unrelated interview, Benjamin also mentions Bachelor In Paradise as the movie in question.
Just a minor correction. It was Bachelor In Paradise opening in London when they decided to get married, not Where The Boys Are. Boys was released in the UK in May 1961. Benjamin and Prentiss were married on October 26, 1961.
With all due respect, I disagree. I would definitely replace Strange Bedfellows and A Very Special Favor with the far superior Lover Come Back and Come September.