I think you could called it a veiled reference to sex. When Cole Porter was writing, he wouldn't have got away with an explicit reference to sex but for many years, children/young people have been taught about sex with the phrase "the birds and the bees". The opening lines of this song, for me, suggest that that is what he was getting at, but then he goes on write "Let's do it, let's fall in love". So I think the lyrics put the suggestion of sex into the listener's mind but it is then tempered by the fact that "it" later seems to refer to simply "falling in love".
Student or Learner