t-rex
Explanation
The Joke
A woman in lingerie tells her partner to "come at me like a T-Rex." The man, now shirtless, asks "So... specifically?" trying to figure out what she means. The woman clarifies with an unexpected answer: "Dead, but still rock hard" -- referring to the fact that T-Rex dinosaurs are extinct (dead) and their remains are now fossils (rock hard). In the final panel, the man awkwardly asks "Can I go home now, please?" while the woman grins at him, delighted by her own pun.
The comic sets up what appears to be a standard bedroom role-play scenario, then subverts it with a paleontology-based pun. The phrase "come at me like a T-Rex" sounds like it should be about ferocity or dominance (the T-Rex being the iconic apex predator), but the woman's actual intent is to deploy a nerdy double entendre.
The Humor
The humor works on multiple levels. First, there is the bait-and-switch from a seemingly steamy scenario into a groan-worthy pun. Second, "dead but still rock hard" works as both a description of a fossilized dinosaur and as a darkly absurd sexual instruction. The man's discomfort in the final panel mirrors the audience's reaction to the pun -- it is simultaneously clever and terrible. SMBC frequently uses this structure where one character is far too pleased with a joke that torments everyone around them.