Heteroflexibility refers to engagement in minimal homosexual activity, not exclusively heterosexual activity. The term is used to distinguish between those that identify as bisexual.
More About Heteroflexibility
Although bisexual individuals as well as heteroflexible individuals both engage in both heterosexual and homosexual activity, heteroflexible individuals engage in minimal homosexual activities and the behavior is largely situational. Bisexual individuals, generally, will engage in either homosexual or heterosexual activity freely. An individual who is heteroflexible might only engage in homosexual activities in particular or specific situations.