Heteroflexible or Elasexual is used to describe an individual who is generally straight/heterosexual and primarily experiences attraction to the "opposite" gender/dissimilar genders, but is occasionally attracted to the same or similiar gender(s).
Another common way to use describe term is individuals that are heterosexual, but from time to time can experience attraction to the same gender. When they do experience attraction to the same gender, the attraction has often nothing to do with the individual's gender, only the individual. This definition is common for heterosexual individuals who have a partner that have come out as trans, whom they still love and want to be together with even though they now are the same gender.
Individuals who identify as heteroflexible may identify with this label only, or they may also identify as straight. Heteroflexible individuals may also identify as or be considered bisexual (otherwise multisexual), however for some heteroflexibles, their attraction to the same gender is so uncommon that they do not consider bisexual to be an accurate description of their orientation in most situations.
The homosexual counterpart to heteroflexible is Homoflexible.
The Kinsey Scale is a way to determine one's sexual preferences through a test. Once one takes the test, they will be given a score from 0-6; 0 being exclusively heterosexual, 6 being exclusively homosexual. Heteroflexible individuals will usually fall at about a 1 on this scale.