Someone who is attracted primarily or exclusively to members of the same gender is characterized as gay or homosexual, though the latter word has largely fallen out of use.
Someone who is attracted primarily or exclusively to members of the opposite gender is characterized as straight or heterosexual.
Someone who has strong, viable attraction for people of both genders is characterized as bisexual or pansexual.
Sexual orientation has replaced the archaic term "sexual preference," which was predicated on the false belief that all people can choose to be viably attracted to people of either gender. People who have the ability to do so are, as noted above, bisexual.