"I learned it by watching you!" is from an old anti-drug PSA:
It's a common joke used in parody to make light of dumb situation by treating it seriously.
The underlying theme of "The Cabin in the Woods" is that these otherwise well-rounded people are all reduced to the prototypical stereotypes of the standard horror film.
"Thor"/Christ Hemsworth is fulfilling the role of the "meathead jock" for the horror genre, but the scene shows that in actuality he's a very intelligent, well-read, thoughtful, helpful, and respectful gentleman -- shown by how he finally points-out, non-sexually, that she isn't wearing any pants.
He's offering Dana (Kristen Connolly) some well-informed advice, and when Jules (Anna Hutchison), his girlfriend, is intelligently joining in on the conversation, despite the "expectation" that she fulfils the role of "dumb blonde girl", they cleverly and convincingly (to be satirical) make light of the situation by pretending that it's bad for her (Jules) to know how to read.
It's a mockery of the stereotypes of the horror genre and the heteronormative patriarchy of anti-feminist* sociocultural traditions.
- They're not really upset.
- It's satire.
- It's in reference to a semi-well-known PSA in the U.S. popular culture.
*Anti-feminist: not believing that feminine people are full, equally extant persons.