According to Wikipedia, Doris Day began to be referred to in this way in the mid–1960s, when "the sexual revolution of the baby boomer generation had refocused public attitudes about sex. Times changed, but Day's films did not." Wikipedia's only real citation for this is behind the Daily Telegraph's paywall, but there are plenty of other websites that back it up.
It was in 1965 that the American pianist, composer and wit Oscar Levant wrote in his memoir, "My last picture for Warners was Romance on the High Seas. It was Doris Day's first picture; that was before she became a virgin." This quote (invariably paraphrased) is often misattributed to Groucho Marx.
© Haydn Thompson 2022