Language
Explanation
The Joke
The comic's header reads: "Annoyance: People who use social justice language strictly for personal benefit." A man tells a woman: "Women make up 52% of the population but only 50% of our marriage. If we added just one more woman, we'd be so much more representative, so much more beautifully emblematic of this diverse nation."
The man is using the language of diversity, equity, and representation -- concepts typically associated with social justice advocacy -- to argue that his marriage should include an additional woman. He frames his desire for polygamy (or at least a threesome) as a matter of fair demographic representation, co-opting progressive rhetoric for an entirely self-serving purpose.
The Humor
The humor lies in the transparent bad faith of the argument. The man deploys statistics, appeals to representation, and social justice vocabulary with superficial competence, but the goal is absurdly selfish. The comic satirizes people who weaponize progressive language for personal gain, using the most obvious and ridiculous possible example to make the pattern unmistakable. The woman's expression suggests she is not remotely fooled by this argument, which makes his attempt all the more pathetic.