Horses were a symbol of sexuality in medieval culture. The horse in "The Reeve's Tale" is especially a symbol of sexuality because he's a stallion, released into a field with a bunch of mares. He eagerly runs off to copulate with a joyous cry of "wehee!" (212). The difficulty the clerks have in catching him probably represents the difficulty of containing the sexuality of young and lusty people (like these two clerks). It's also a foreshadowing of the rampant sexuality soon to come in the miller's house that night, when John and Aleyn have sex with Symkyn's wife and daughter. The horse's release might be the story's way of signaling that a period of sexual freedom has begun. It's also a way of signaling that what we're reading here is a bawdy story – one that's all about sex, sex, and more sex.
A few of the non-sexual events that occur in "The Reeve's Tale" are described in such a way that they make us think of sex. Remember that moment when John proposes to stand right by the "hopur" to see how it goes "to and fra," and Aleyn declares his intention to stand below it and watch "how that the mele falles doun / Into the trough" (188-189)? Well we might compare the up-and-down / in-and-out motion to the act of sex.
Another moment like this occurs when Aleyn and Symkyn fight. The narrator tells us that "they walwe as doon two pigges in a poke; / And up they goun, and doun agayn anon" (424-425). Coming after the sexually-charged events that have just occurred, it's not just the dirty-minded who see a likeness to sex in this description. All of this sexual imagery is likely present to signal to the reader that this is a bawdy story. In this kind of story, everything, even something as mundane as grinding corn into flour, is somehow related to sex.