Someone who has no preference, in sexual partners, in that they don't identify as being exclusively gay or straight. Usually that they are capable of relations, with both the opposite sex, as well as the same gender.
Generally referred to as being Bi Curious, or Bi Sexual.
However, the term Heteroflexible usually implies that the person is straight, but curious to experience a relationship with a male, or is open to it.
Homoflexible is the opposite, in which a gay person is open to the possibility of a relationship with someone of the opposite sex, though their preference, is other's of the same gender.