Used as a response to point out an unintentional double-entendre in which the original speaker could be taken as intending to perform some act that would be more appropriate for a sex, kink, or other "special" party.
Person A: "Hey, I'm going out to eat. Wanna come?"
Person B: "Sure, why not?"
Person A: "Alright."
Person B: "Um, could you buy my food?"
Person A: "...it ain't that kind of party."