Simply put, sexuality is complicated.

In 1948, famed American sexologist Alfred Kinsey and his colleagues published "Sexual Behavior in the Human Male." The book included the Heterosexual-Homosexual Rating Scale, more commonly known as the "Kinsey Scale." Based on their subjects' sexual histories, Kinsey's team made a scale from 0 to 6, with 0 being exclusively heterosexual, 6 exclusively homosexual, and 3 being equal parts of the two.

Demonstrators march for marriage equality in Mexico City. Photo by Yuri Cortez/AFP/Getty Images.

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