It ain't always easy having erectile dysfunction, but it sure as hell ain't hard.
Erectile Dysfunction Difficulty
Explanation
The joke uses wordplay on the phrase "it ain't hard" which can refer both to the difficulty of a situation and to the physical state of having an erection. The punchline is that while dealing with erectile dysfunction is tough, the state of 'not being hard' is definitely not hard to achieve for someone with that condition.