quote
Explanation
The Joke
A woman at a bar propositions a man by asking, "Want to have 'sex' tonight?" -- with the word "sex" in quotation marks. The man, instead of responding to the proposition itself, launches into a grammatical analysis: "Are you one of those people who uses quotation marks for emphasis, or are you using quotations to imply a hidden and terrible form of the word contained inside?" He then adds, "Because if it's the first thing, ewwwwww."
The joke inverts expectations by having the man be disgusted not by the possibility of strange or terrible sex, but by the grammatical error of using quotation marks for emphasis. The use of scare quotes around a word typically implies that the word should not be taken at face value -- that the thing being described is not really what it claims to be. So "sex" in quotes could imply something that is called sex but is actually something else entirely (and presumably worse). The man, however, finds the misuse of punctuation far more offensive than any implied deviance.
The Humor
This is a classic grammar-nerd joke that plays on the well-known pet peeve of quotation marks being misused for emphasis (a common sight on handwritten signs like: Fresh "Fish" Today). The comic escalates the absurdity by having the man express active disgust ("ewwwwww") at the grammatical error rather than at the implications of the scare quotes, suggesting he would actually prefer hidden and terrible sex to poor punctuation. It satirizes the priorities of pedantic grammar enthusiasts who care more about proper usage than about the actual content being communicated.