fantasy
Explanation
The Joke
A couple is in bed and one partner asks the other to share their "darkest fantasy" and to "force it upon" them. The other partner hesitantly says "I dunno... it's just pretend, come on, it'll be fun." They begin roleplay, and the "fantasy" turns out to be: the partner pulls out what appears to be powdered sugar and starts coating the other person, who screams in surprise. The partner with the powder declares it is "powdered sugar!" and the other asks "Do I get to eat it too?" The response: "It got too real." In the final panel, the messy aftermath reveals "Awkward Engagement" -- suggesting this strange scenario somehow led to a marriage proposal.
The humor comes from the bait-and-switch setup. The audience is led to expect a risque sexual fantasy, given the bedroom setting and the suggestive "darkest fantasy" framing. Instead, the "fantasy" is something absurdly innocent and bizarre -- being covered in powdered sugar. The escalation from "darkest fantasy" to powdered sugar is the core comedic reversal.
The Humor
The comic plays on the common trope in media and real life where couples nervously share intimate fantasies, building tension and anticipation for something scandalous. Weinersmith subverts this entirely by making the fantasy simultaneously innocent and weird -- not sexual at all, but still somehow embarrassing and "too real." The additional layer of the final panel suggesting this led to an engagement adds another absurd twist, implying that sharing your true weirdness with someone (even if it is just powdered sugar) is the ultimate form of intimacy and commitment.