With the question of gay marriage looming large in states across the nation and in the GOP nominee scramble, Beacon Books on Jan. 30 will release a new title by scholar-author Hanne Blank on the origins of the label we use to describe “traditional” married couples. Here, an excerpt from the introduction of Straight: The Surprisingly Short History of Heterosexuality.

Would that defining “heterosexual” were as simple a problem as defining a particular type of sexual desire or activity. Alas, human sexual interests and behaviors are every bit as ambiguous and complicated as biology and gender. Science has not been able, at this point, to supply a definitive answer to the question of why and how our sexual interests and desires arise. What the available evidence suggests is that sexual desires are partly intrinsic to the individual and partly learned or acquired from culture. How intrinsic and learned desires relate to one another, however, and the actual mechanisms that cause a given person to desire one thing or person but not another, remain an utter mystery.

Many of the things that people frequently find sexually desirable can be linked to reproductive success. Clear skin and good teeth are signs of health. Curvy female hips are a signal of sexual maturity and suggest that a woman will bear children easily. But there is huge variety in what can be found sexually appealing, and quite a bit of it has no meaningful bearing on reproductive success whatsoever. Long lean bodies, short fleshy bodies, pale skin, dark skin, blue eyes, brown eyes, red hair, black hair, moustaches, and hands with long thin fingers all have their ardent partisans, although none of these characteristics are necessary in order to make happy, healthy babies. Why should “gentlemen prefer blondes,” as some assuredly do? Or redheads? Why should women find cleft chins appealing? What’s the percentage in being besotted by freckles? Hard to say. There doesn’t appear to be any obvious biological benefit in it. And, of course, sexual desires are not limited to human beings or their bodies. History tells us that people can and sometimes do feel sexual desire for, among rather a lot of other things, shoes, urine, barnyard animals, latex rubber, and trees. The so-called “reproductive imperative,” in other words, is not necessarily what is driving the bus when it comes to our experiences of sexual desire, not even when what we desire is a human being of a different biological sex than ourselves.

The staggering variety of things we can and do desire is only exceeded by the number of things we do with them. How do we define a sexual activity as being a sexual activity, and what does it mean to engage in one? Historically, the criteria for what constituted “sexual activity” for most scientific purposes have been remarkably narrow, confined solely to the act of penis-in-vagina intercourse. Philosopher Marilyn Frye, in a 1988 essay entitled “Lesbian ‘Sex,’” noted that in most sexological research, this was additionally narrowed to describe only the experiences of males. What this means is that for many decades, for the purposes of biomedical science, the sex act of record consisted of the insertion of an erect penis into a vagina, the thrusting of that penis within the vagina, and the ejaculation of semen from the penis followed by withdrawing the penis from the vagina.

You’d never know it from the insert-thrust-squirt-remove trajectory that serves as a description of this activity, but penis-in-vagina intercourse actually involves two people. Their genitals are very different, they engage in different physical activities during this type of intercourse, and they have different sensory experiences of it. Neither would you guess that human sexual activity included many other options, performable by many different combinations of participants of various biological sexes. This has been a very real problem in both the study and the popular discussion of sexual activity: a single lopsided model of a single activity is held up not just as the baseline from which all else departs, but as the alpha and omega of human sexual behavior. The resulting picture we have developed of sexuality and its workings has historically been severely biased. Surely if we are trying to understand just what sexual activity and heterosexuality have to do with one another, and how that relationship might work, it is incumbent upon us likewise to consider that “sexual activity” doesn’t look the same for everyone. As Lisa Diamond and Michael Bailey have begun to argue, the narrowly male-oriented focus of so much sexological research means that when it comes to sexual orientation, models developed on the basis of male-oriented research might not even be appropriate tools to use to help figure out how sexuality works for women. The implications, insofar as the applicability of our current system of understanding and classifying sexuality, are staggering.

Another way in which our understanding-sexuality toolbox is often lacking concerns the variety of functions that sexual activity fulfills. People often attempt to justify the heterosexual/homosexual scheme on the basis of the fact that while different-sex couples can engage in reproductive sex, same-sex couples cannot. But reproduction is hardly the only reason different-sex couples engage in sexual activity. It cannot be. For purely mechanical reasons because a specific sexual act must be performed by two fertile people of different biological sexes at the right time of the menstrual cycle for conception to even be possible—procreative sexual activity can only possibly account for a small subset of all sexual activity between women and men.

In truth, sexual activity is social activity. Our culture is often loath to recognize this, although we do embrace the idea that sexual activity can be about the social functions of expressing affection and intensifying social and emotional bonds. Indeed, many people believe that sex is only justified by love. But sexual activity has many other social roles to play. It can be a reward, a mode of exchange, a way to affirm loyalty, or an appeasement. It can be a commodity, a way of providing reassurance, and a rite of passage. As a source of pleasure it has few equals. It’s an age-old means of asserting dominance and a visceral mode by which to demonstrate submission. It can furthermore be a means of gaining control, a way to humiliate and violate, and a way to punish. And any given sex act, no matter who engages in it, can and often will involve more than one of these dynamics.

The subjective experience of the erotic and of pleasure is, perhaps unsurprisingly, also enormously variable. It’s not just that desires differ from one person to the next, or that some sexual episodes are transcendent and others are only so-so, but that identical objects or actions can provoke entirely different reactions depending on circumstances. Not everything that is potentially desirable is actually desirable. Not all “sex” is sexy. A lover we once found irresistible becomes repulsive after a nasty breakup. A sex act we enjoyed with one partner may just not do it for us when we try it with another. Some argue that it may not even be appropriate to call some examples of “sexual acts”—rape, for example—sexual at all.

All of this brings us back around to the issue of heterosexuality and what we must take into account if we are going to illuminate it in any way. Human sexuality, as should be clear by now, encompasses much more than the ways that the biological sex(es) or social gender(s) of the people we fancy compare to our own. Whom we choose as erotic-activity partners is just one aspect of what we do sexually. Words like “heterosexual” may hint at, but do not accurately denote, all the complexities (or vagaries, or ambiguities) of an individual’s actual lived experience of sexuality.

Because there is so much inbuilt variability where sexuality is concerned, there are five caveats worth keeping in mind for any exploration of sexual orientation. First, the biological sex and social gender of a prospective partner are only two of many characteristics in which an individual may take a sexual interest, and their relative importance is subjective and variable. Second, sexual desire (what we like or want) and sexual behavior (what we actually do) are not the same thing, and may or may not be related. Third, sexual and/or erotic activity take on considerably more forms than we may be personally accustomed to recognize, and certainly more forms exist overall than are sanctioned by any given culture. Fourth, we have to remember that all sexual activity is social activity, while only a small subset of all sexual activity is also reproductive activity. This means that it behooves us to think about sexual activity first as social, and only consider it as (potentially) reproductive when it actually is. And last, we must bear in mind that the relationships between perception, thought, emotion, and behavior are neither automatic nor consistent. In many cases they are demonstrably affected or directed by culture and socialization. We don’t just want what we want because we want it; we want what we want because that’s what we’ve learned to want.

With this in mind, we can proceed to take a look at the history of heterosexuality. As we should, because whether we like it or not, the idea of heterosexuality is embedded in each of us, in our actions and reactions, our emotional responses, and our intellectual assertions. We can see its distinctive imprint in the things we believe about love, in the ways we pursue pleasure, in the things we expect from our relationships, our work, our government, and our genitals. This concept we call “heterosexuality” doesn’t just shape our sex lives; it shapes the ways we understand the world to work and, consequently, the ways it does. Heterosexuality reaches too far beyond the merely personal, and in too many profound and pervasive ways, for us to write it off as a simple matter of biology or nature or even Divine plan. It cannot be reduced to economics, the search for pleasure, or even to true love. It certainly cannot be reduced to a few checkboxes on a clinic form. All of these things may play a part in what we think of when we think about heterosexuality, but none of these things are heterosexuality.

Excerpt from Straight by Hanne Blank. Copyright © 2012 by Hanne Blank.

Reprinted by permission of Beacon Press.