note: you have to know what a jockey is to understand the joke, so a jockey is a person who rides the racehorses....
So these three friends are having a guy's night out at the bar. after a few drinks, the first one confesses, "i think my wife is cheating on me with the mail man." the other two are surprised for a few seconds then say, "oh really, what makes you think that?" he replies, "well, i found a large bag full of other people's mail."
then the second one also admits, "i think my wife is cheating on me with the gardener." the other two are surprised for a few seconds then say, "oh really, what makes you think that?" he replies, "well, i found a bunch of dirty gardening tools that weren't mine under the bed."
then the third one sighs and says, "i think my wife is cheating on me with a horse." the other two are completely surprised and as, "why do you think that?" he replies, "i found a jockey under my bed"