Bisexuality is a sexual behavior or an orientation involving physical or romantic attraction to both males and females. It is one of the three main classifications of sexual orientation, along with a heterosexual and a homosexual orientation. Individuals who lack sexual attraction to either sex are known as asexual. Bisexuality does not require that a person be attracted equally to both sexes. People who have a distinct but not exclusive preference for one sex over the other may still identify themselves as bisexual.