I was insecure about sex. I've grown more secure. I used to use the amateur phylactics, and I only use the prophylactics now.

The line is built as a classic Steve Martin-style one-liner: a mock-confessional setup (“I was insecure about sex…”) followed by a punchline that hinges on near-homophones and faux-technical diction. By contrasting “amateur phylactics” with “prophylactics,” the joke pretends to chart personal growth and sexual confidence, but the “growth” is actually a linguistic slip—an absurd misuse of a learned-sounding word. The humor comes from the speaker’s self-serious tone, the escalation from vulnerability to supposed sophistication, and the sudden realization that the narrator’s insecurity is intellectual/linguistic as much as sexual.