Heteroflexible

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, they very rarely have tendencies to be attracted to the same gender. People who identify as heteroflexible may identify with this label only, or they may identify as straight. Heteroflexible people can be considered bisexual or otherwise multisexual, however for some their attraction to the same gender is so rare that one may not consider bisexual to be an accurate description of their orientation in most situations.

Another common way to use this term is people 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 person's gender, only the person.

This definition is common for heterosexual people who has a partner that have come out as trans, which they still love and want to be together with even though they now are the same gender.

Kinsey Scale
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.

Resources

 * 1) Pride Flags - Heteroflexible | Deviantart