lie
Explanation
The Joke
A child walks in on their parents in a compromising situation and asks, "Mom? Dad? What were you doing to each other?!" The father panics, thinking "Think of a lie think of a lie think of a lie..." but then blurts out the most explicit, oversharing truth possible: "We were having conventional highly affectionate penis-in-vagina sex!" The father is shown wearing what appear to be devil horns and holding a pitchfork, suggesting the encounter was anything but "conventional."
The classic sitcom setup is a child catching their parents having sex, and the parents scrambling to invent a cover story. Here, the father's panicked attempt to lie results in him telling a version of the truth -- but the visual details (costume accessories suggesting something kinky) reveal that even his "honest" answer is itself a lie, just in the opposite direction. He is downplaying rather than covering up entirely.
The Humor
The comedy works in layers. The first layer is the simple inversion of the trope: instead of making up an innocent excuse, the father blurts out an explicit description of sex. The second, subtler layer is the irony that even this seemingly over-honest answer is a lie -- the devil horns and pitchfork suggest the activity was far from "conventional." So the father, in his panic to think of a lie, accidentally told one anyway, just not the kind he intended.