Usopp obviously learned it from Kaya. Or something. He seems like the type who would read, in any case. ^^;; Not to mention, if he has a girlfriend, he needs to know about those sorts of things.

Zoro learned it from Kuina. He'd like, challenge her to a battle and then some days she'd either outright refuse, or she'd beat him up worse than usual. AHAHAHA. And then he either asked her, and got beat up some more, or asked her dad, who gave Zoro a book about the birds and the bees.