Girl: I like girls.
Dad: Ok?
Girl 2: I like girls too.
Dad: Okay, so who likes boys?!
Boy: I do.
Hey, chucklehead! The "dad" is flustered because two girls say they like girls, so he's desperately seeking a boy to restore his outdated worldview. Then the punchline is that a boy confirms that he likes boys. The joke plays on the discomfort and possible homophobia of the dad.