Explain SMBC — the wiki for Saturday Morning Breakfast Cereal

This comic shows a man in bed with a woman, dramatically proclaiming "Oh... there must be kryptonite hidden somewhere here. Uh... Damn you, Lex Luthor! You win this ro--" before the woman cuts him off with "Stop. Just stop." The caption below reads: "Later in life, Superman refuses to admit he has erectile dysfunction."

The joke imagines Superman dealing with a very human problem -- erectile dysfunction -- but refusing to acknowledge it by blaming his failure on kryptonite, the one substance that can weaken him. Rather than confronting an embarrassing medical issue, he constructs an elaborate excuse involving his arch-nemesis Lex Luthor, as if his inability to perform is part of a supervillain's plot. The humor comes from the collision of superhero mythology with mundane middle-aged reality, and from the universal human tendency to make excuses rather than admit vulnerability.