Heterosexuality is romantic or sexual desire for, or sexual activities with, persons of the opposite sex. It is overwhelmingly the most common sexual preference or orientation. Heterosexual behaviour (notably coitus) is responsible for the reproduction of most species, including humans.
According to some studies, notably Sexual Behavior in the Human Male (1948) and Sexual Behavior in the Human Female (1953) by Dr. Alfred C. Kinsey, note that when asked to rate themselves on a continuum from completely heterosexual to completely homosexual, and when the individuals behavior as well as their identify is analyzed, the majority of people appear to be at least somewhat bisexual. Most people have some attraction to either sex, although usually one sex is preferred. According to Kinsey, only a minority (5-10%) can be considered fully heterosexual or homosexual. Conversely, only an even smaller minority can be considered "fully" bisexual. See Kinsey Reports.
Heterosexuality is colloquially referred to as being "straight", although that term can also mean anyone whose sexuality could be described as orthodox. For example, people who practice BDSM use the term to refer to anyone who does not.