if-then
Explanation
The comic plays on the logical asymmetry of conditional ("if-then") statements. A couple has apparently agreed that they should "incorporate eating into sex." One partner has taken his pants off while the other is eating lasagna, and she says she's getting "such mixed signals" from him. The caption at the bottom spells out the joke explicitly: 'Turns out "we should incorporate eating into sex" does not imply "we should incorporate sex into eating."'
The humor is rooted in formal logic. In propositional logic, "if A then B" does not entail "if B then A" -- that's the fallacy of affirming the consequent (or simply confusing a conditional with its converse). The comic maps this logical principle onto a relationship scenario: one partner interprets the agreed-upon statement as bidirectional, leading to an awkward misunderstanding at the dinner table. The title "if-then" is a direct reference to this logical structure.
There is also a layer of relationship comedy here: the absurdity of someone showing up pantsless at a lasagna dinner because of a misapplied logical inference, and the other partner's bewildered reaction, plays on the common trope of couples miscommunicating about intimacy.