WATCH: Zach Wahls on Gay Marriage, Bullying, and Kim Kardashian

Last year, Zach Wahls rose to national attention when a video of him defending his gay moms before the Iowa House of Representatives became YouTube's most viewed political video of 2011. Now the 20-year-old writer, activist, and entrepreneur shares the story of his family in a compelling new book, My Two Moms: Lessons of Love, Strength, and What Makes A Family. Wahls visited David Letterman Tuesday and discussed the book, being bullied growing up, and the irony of a gay threat to the "sanctity" of marriage in a world shared with Kim Kardashian.

Watch it below.