According to the American Psychological Association (APA), sexual orientation, biological sex, gender identity, and social sex, are the four elements which comprise human sexuality. Sexual orientation, as defined by the APA, is "distinguished by an enduring emotional, romantic, sexual or affectionate attraction to individuals of a particular gender." The three commonly recognized aspects of sexual orientation are heterosexuality (sexual attraction to people of the opposite gender), bisexuality (sexual attraction to people of both the male and female gender), and homosexuality (sexual attraction to members of the same gender). Sexual orientation differs from sexual behavior in that it involves an individual's feelings and perception of their own sexuality. On the other hand, sexual behavior--or the act of sex--may or may not express an individual's feelings or self-concept.

The origins of sexual orientation have puzzled philosophers, theologians, and ordinary people for thousands of years. In a few cultures...