"The Miller's Tale" is all about sex: who's having it, who's not, how they're managing to have it, and the consequences of it. With the contrast in age between John and Alisoun, the tale raises the question of whether the youthful and the old are an acceptable sexual pairing. And with the very different courting styles of Nicholas and Absolon, it explores the techniques a lover might use to obtain his desires. Although medieval romances (and many other genres) also dealt with their characters' desires for sex, "The Miller's Tale" does so in explicit, at times even crude language, in effect calling a spade a spade in the way that other genres don't.
"The Miller's Tale" suggests that a young person is not an appropriate sexual partner for an older person. It portrays Alisoun and Nicholas's affair as just punishment for John's inappropriate marriage.
The characterization of Alisoun using barnyard animal imagery draws our attention to the way in which sex is a natural process that breaks down the separation between animals and humans.