Heterosexuality is not a social construct, it is the cornerstone of reproduction and the continuation of the species, it is the normative that has driven human evolution.
... Sure, it is. How exactly you define heterosexuality is certainly a social construct. This is why it is so difficult to define heterosexuality (and homosexuality) consistently in studies. Of course, heterosexual sex is necessary to propagate the species, but that doesn't say anything about whether
heterosexuality as a discrete identity is a social construct or not.
You should read George Chauncey's
Gay New York: Gender, Urban Culture, and the Making of the Gay Male World, 1890-1940:
This book argues that in important respects the hetero-homosexual binarism, the sexual regime now hegemonic in American culture, is a stunningly recent creation. Particularly in working-class culture, homosexual behavior per se became the primary basis for the labeling and self-identification of men as "queer" only around the middle of the twentieth century; before then, most men were so labeled only if they displayed a much broader inversion of their ascribed gender status by assuming the sexual and other cultural roles ascribed to women. The abnormality (or "queerness") of the "fairy," that is, was defined as much by his "woman-like" character or "effeminacy" as his solicitations of male sexual partners; the "man" who responded to his solicitations - no matter how often - was not considered abnormal, a "homosexual," so long as he abided by masculine gender conventions. Indeed, the centrality of effeminacy to the representation of the "fairy" allowed many conventionally masculine men, especially unmarried men living in sex-segregated immigrant communities, to engage in extensive sexual activity with other men without risking stigmatization and the loss of their status as "normal men."
[...]
Heterosexuality had not become a precondition of gender normativity in early-twentieth-century working-class culture. Men had to be many things in order to achieve the status of "normal" men, but being "heterosexual" was not one of them.
[...]
In a culture in which becoming a fairy meant assuming the status of a woman or even a prostitute, many men, like the clerk, simply refused to do so. Some of them restricted themselves to the role of "trade," becoming the nominally "normal" partners of "queers" (although this did not account for most such men). Many others simply "did it," without naming it, freed from having to label themselves by the certainty that, at least, they were not fairies. But many men aware of sexual desires for other men, like the clerk, struggled to forge an alternative identity and cultural stance, one that would distinguish them from fairies and "normal" men alike. Even their efforts, however, were profoundly shaped by the cultural presumption that sexual desire for men was inherently a feminine desire. That presumption made the identity they sought to construct a queer one indeed: unwilling to become virtual women, they sought to remain men who nonetheless loved other men.
The efforts of such men marked the growing differentiation and isolation of sexuality from gender in middle-class American culture. Whereas fairies' desire for men was thought to follow inevitably from their gender persona, queers maintained that their desire for men revealed only their "sexuality" (their "homosexuality), a distinct domain of personality independent of gender. Their homosexuality, they argued, revealed nothing abnormal in their gender persona. The effort to forge a new kind of homosexual identity was predominantly a middle-class phenomenon, and the emergence of "homosexuals" in middle-class culture was inextricably linked to the emergence of "heterosexuals" in that culture as well. If many working men thought they demonstrated sexual virility by playing the "man's part" in sexual encounters with either women or men, normal middle-class men increasingly believed that their virility depended on their exclusive sexual interest in women. Even as queer men began to define their difference from other men on the basis of their homosexuality, "normal" men began to define their difference from queers on the basis of their renunciation of any sentiments or behavior that might be marked as homosexual. Only when they did so did "normal men" become "heterosexual men." As Jonathan Katz has suggested, heterosexuality was an invention of the late nineteenth century. The "heterosexual" and "homosexual" emerged in tandem at the turn of the century as powerful new ways of conceptualizing human sexual practices."
In short, "heterosexuality" becoming a part of the definition of "normal" manhood and masculinity being defined in part as the exclusive interest in women and not men is something that was only really codified by the late 1930s and early 1940s. Before this, the dichotomy was between "normal men" and "fairies", and normal men could have sex with men as long as they played the "proper" role.
