Wild
Explanation
The Joke
A man in an intimate setting tells a woman, "Baby, I could do you nonstop forever." She responds enthusiastically, "You absolute wild man!" The caption below reads: "The sexiest taboo to violate is the Second Law of Thermodynamics."
The phrase "do you nonstop forever" sounds like a sexual boast, but the caption reframes it as a physics claim. Doing anything "nonstop forever" would require a perpetual motion machine, which violates the Second Law of Thermodynamics. This law states that entropy in a closed system always increases, meaning that energy inevitably dissipates and no process can continue indefinitely without additional energy input. The man is not just making an impressive sexual promise -- he is claiming to defy a fundamental law of the universe.
The Humor
The joke works by placing a physics violation inside a romantic/sexual context. The woman's enthusiastic response ("You absolute wild man!") reads as if she is excited by his sexual stamina, but in light of the caption, she may actually be thrilled by the thermodynamic impossibility he is proposing. The caption's deadpan declaration that violating the Second Law is "the sexiest taboo" is funny because it treats a physics principle as something that could be erotically transgressive, applying the language of sexual rebellion to entropy. It is a quintessential SMBC joke -- finding the intersection between physics and human desire and mining it for absurdist humor.
References
The Second Law of Thermodynamics, formulated in the 19th century by Rudolf Clausius and Lord Kelvin among others, states that the total entropy of an isolated system can only increase over time. It implies that perpetual motion machines of the second kind are impossible and that all natural processes are irreversible. It is sometimes colloquially summarized as "you can't break even" in energy transactions.