Heteroflexibility is a form of a sexual orientation or situational sexual behavior characterized by minimal homosexual activity in an otherwise primarily heterosexual orientation that is considered to distinguish it from bisexuality. It has been characterized as "mostly straight". Although sometimes equated with bi-curiosity to describe a broad continuum of sexual orientation between heterosexuality and bisexuality, other authors distinguish heteroflexibility as lacking the "wish to experiment with ... sexuality" implied by the bi-curious label. The corresponding situation in which homosexual activity predominates has also been described, termed homoflexibility.
National surveys in the U.S. and Canada show that 3 to 4 percent of male teenagers, when given the choice to select a term that best describes their sexual feelings, desires, and behaviors, opt for "mostly" or "predominantly" heterosexual. With "100% heterosexual" being the largest assumed identity, "mostly-heterosexual" was the first runner up in self-identification. Of the 160 men interviewed for a study in 2008 and 2009, nearly one in eight reported same-sex attractions, fantasies, and crushes. The majority had these feelings since high school; a few others developed them more recently. And in a national sample of young men whose average age was 22, the "mostly straight" proportion increased when they completed the same survey six years later. An even higher percentage of post-high-school young-adult men in the U.S. and in a handful of other countries (including New Zealand and Norway) make the same choice.
As of 2010, most studies of heteroflexibility have focused on young men and women, especially white women in the college environment. Research suggesting the influence of prenatal androgen exposure on female sexual identity places heteroflexibility on a continuum with bisexuality and lesbianism. Other studies have focused on social origins for the behavior, such as the shifting media presentation of bisexuality or the "socialization of the male interloper fantasy" in which a man is invited into a lesbian relationship as a third partner.
Unlike "bisexual until graduation" and similar pejoratives, heteroflexibility is typically considered to have a positive connotation, and is often a self-applied label, although use of the term as a pop-culture slur has been attested.