All men make mistakes, but married men find out about them sooner.

The joke hinges on a contrast between making mistakes (universal human fallibility) and being made aware of them (social accountability). “Married men find out about them sooner” implies that marriage creates an intimate, continuous audience: a spouse observes one’s habits closely and is more likely to point out errors immediately. Beyond the punchline, the remark gestures at how partnership reduces the privacy in which self-deception can flourish; it also reflects a traditional comic stereotype of the sharp-eyed, corrective spouse. Read more generously, it suggests that close relationships accelerate feedback—sometimes irritatingly, sometimes helpfully—forcing quicker recognition of one’s shortcomings.