Explain SMBC — the wiki for Saturday Morning Breakfast Cereal

In the first panel, a father (shown in silhouette with a unicorn-like horn) tells his son that he and the boy's mother do not believe in premarital sex. The son angrily retorts, "Well, I don't believe in YOU, Dad!" In the final panel, we see the father is revealed to be a unicorn -- a mythological creature that, famously, people do not believe in.

The joke operates on a double meaning of "believe in." The father uses it in the sense of moral disapproval ("we don't believe in premarital sex"), while the son's retort uses it in the literal sense of believing in something's existence. The punchline reveals that the son's response is not just a petulant comeback but a factually meaningful statement, since his father is a unicorn. The mother standing next to the unicorn father in the final panel adds to the absurdity of the domestic scene.