I think the Mr. Boffo comic below gives us quite a rich picture (no pun intended) of several different facets of traditional gender binaries:
The reason the joke 'works' (if it does) is that door-to-door salesmen (are there still door-to-door salespeople, other than those peddling religion?) can gain favor from women who answer the door by pretending that those women aren't, in fact, women at all, but girls--because there's nothing worse if you're female than looking old enough to, say, own your own house. Not only can they gain favor by doing so, but it's complementary. And it's 'funny', see, because salesmen can't do the same thing with men because it's not complimentary, generally, to call a man a boy.
On the other hand, here's a comic which implicitly acknowledges the Ultimate Power that comics have in reinforcing gender stereotypes:
Plus, it sort of offhandedly is snarky about the old testament, as if Moses (or perhaps another prophet?) would still be walking around, lost, today. Which is a bonus in my book, even if it also reinforces the male stereotype by imagining that a guy would rather lead his people through the desert forever than ask to stop and ask for directions...