Heterosexuality is exclusive romantic or sexual desire for, or sexual activities with, persons of the opposite sex. Heterosexual sex, or straight sex, is sex between heterosexuals. It is overwhelmingly the most common sexual orientation. Heterosexual sexual intercourse is responsible for the reproduction of most animal species, including humans.
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 (i.e.,bondage and discipline, domination and submission, and sadism and machochism) sometimes use the term to refer to anyone who does not (though the term vanilla is more common).
Some studies, notably Sexual Behavior in the Human Male (1948) (ISBN 0253334128) and Sexual Behavior in the Human Female (1953) (ISBN 025333411X) by Dr. Alfred Kinsey, note that when asked to rate themselves on a continuum from completely heterosexual to completely homosexual, and when the individual's behavior as well as their identity 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, therefore, only a minority (5-10%) can be considered to be fully heterosexual or homosexual. Conversely an even smaller minority can be considered fully bisexual, if that term is defined as having no preference for one sex over another.