Explain SMBC — the wiki for Saturday Morning Breakfast Cereal

A child tells his father there's a monster in the closet. The father goes to check, and the reveal shows a literal monster sitting on the child's bed with the child, saying "I only watch gay porn for the plots!" The monster replies skeptically, "Suuuure you do."

The comic plays on the classic "monster in the closet" setup but subverts it in two ways. First, instead of the monster being a threat, it's just casually hanging out with the kid. Second, and more centrally, the joke is a double play on the phrase "in the closet" -- the monster was literally in the closet, but the punchline suggests a figurative "closet" scenario as well, with the monster making a stereotypical denial about his viewing habits and the child's skeptical response mirroring the way people react to transparent self-deception. The humor is in the unexpected domesticity of the situation and the absurd pairing of a child's bedtime fear with very adult comedy.