He wanted to mark his terf.
Explanation: This joke is a play on words, referencing J.K. Rowling’s controversial stance on gender issues, often labeled as “TERF” (trans-exclusionary radical feminist). The punchline, “He wanted to mark his terf,” is a pun where “terf” sounds like “turf” (as in marking territory). The humor comes from the clever double meaning, combining a joke about a dog marking its territory with a subtle reference to the ongoing debates around J.K. Rowling’s views.