From his mentor Albert the Great, who wrote a massive study of zoology, Thomas learned how comparative animal studies could support the natural law of marriage. Among those animals whose offspring could survive and flourish without the care of both parents, Thomas observed, there was no need for the pair-bonding of a father and mother for the care of their offspring. But among those birds and other animals whose offspring need the care of both parents, there tends to be a natural bond between the parents who jointly care for the offspring. Human beings belong to this latter group of animals.
From this, Thomas infers that while marriage is a distinctly human institution, marriage in rooted in the natural animal instincts for parental care of children and conjugal bonding of husband and wife. He also infers from this comparative animal biology that homosexuality must be unnatural for two reasons. First, nonhuman animals do not engage in homosexual conduct. Second, homosexuality does not lead to procreation and parental care of children (Summa Theologica, I-II, q. 30, a. 3; q. 31, a. 7; q. 94, a. 3, ad 2, q. 94, a. 6; II-II, q. 154, aa. 11-12)..
We now know, however, that Thomas was mistaken about both of these points. Scientists have observed homosexual behavior in 471 animal species--167 species of mammals, 132 species of birds, 32 species of reptiles and amphibians, 15 species of fishes, and 125 species of insects and other invertebrates (Bagemihl 1999, 673). Scientists have also observed that same-sex pairs have successfully reared young in at least 20 species. In some cases, one or both partners are the biological parent(s) of the young they raise together. In other cases, the partners adopt and care for young without being the biological parents (Bagemihl 1999, 23-26). Moreover, in some cases, the same-sex couples seem to be more successful in their parenting than opposite-sex parents.
We also now know that homosexuality is biologically natural in that it arises through the interaction of many biological factors in the early development of fetuses and children--genes and sex hormones shape the body and the brain in early life so that people are naturally predisposed to become heterosexual, bisexual, or homosexual. Monozygotic (identical) twins are more concordant in their sexual orientation than dizygotic (fraternal) twins, which clearly shows a genetic contribution to homosexuality That the concordance between monozygotic twins is about 50% suggests that while there is a genetic influence, there are also other biological factors involved. And while there is no single "gay gene," there are probably many different genes interacting with one another in various ways that influence sexual orientation (Poiani 2010, 55-96). Explaining the biology of animal homosexuality requires a complex multicausal model (Poiani 2010, 401-425).
If homosexuality is biologically natural, does that support the liberty of homosexuals as a natural right? At the website for "LGBT Science," you can see how the LGBT community appeals to the biology of homosexuality to show that homosexuality is natural and thus should be protected as a matter of individual liberty. Similarly, at the end of his book surveying the science of animal homosexuality, Aldo Poiani declares as his "personal ethical stance" his support for "the fundamental human right of homosexuals, bisexuals, transsexuals and intersex people to express their own self with respect to the laws of the land" (425).
And yet Poiani admits that his ethical stance cannot be rooted in his scientific study of homosexuality, because he accepts the idea of the naturalistic fallacy or the is/ought problem. "The findings of scientific research aim at being descriptive of nature and not prescriptive of how we should behave in an ethical sense" (424).
Nevertheless, Poiani clearly implies that there are ethical implications to his argument that homosexuality is not a pathology.
"I regard the potential development of homosexuality in response to stresses experienced during early development as an adaptive response, not a pathology in need of a cure. My appreciation of Freud's insight that early stresses may contribute to cause homosexuality stops at the point where psychoanalysis concludes that therefore homosexuality is a pathology in need of treatment. Adaptive responses have evolved to maintain survival and direct (via bisexuality) or indirect (via helping relatives) reproductive success under challenging environmental (e.g. social) conditions. If so, then homosexuality is adaptive and not a pathology" (422).Poiani is pleased that the American Psychiatric Association voted in 1973 to remove the classification of homosexuality as a pathology of sexuality in its Diagnostic and Statistical Manual of Mental Disorders (DSM). But he is disturbed that the fourth edition of the DSM includes "Gender Identity Disorder," which is identified as a mental disturbance from a "strong and persistent cross-gender identification." Among children, the disturbance is manifest in "(1) repeatedly stated desire to be, or insistence that he or she is, the other sex; (2) in boys, preference for cross-dressing or simulating female attire; in girls, insistence on wearing only stereotypical masculine clothing; (3) strong and persistent preferences for cross-sex roles in make-believe play or persistent fantasies of being the other sex; (4) intense desire to participate in the stereotypical games and pastimes of the other sex; (5) strong preference for playmates of the other sex."
Poiani objects to the assumption here of "a bipolar concept of masculinity and femininity." He explains:
"What I show in this book, and what other authors have shown in their published works, is that gender roles in humans and other social mammals with biparental care are not static and rigidly organized into clearly masculine traits associated with males and feminine traits associated with females. As far as behavior is concerned, there are differences but also great regions of overlap between the sexes in both birds and mammals, including humans" (423).Poiani suggests that the distress felt by those identified as showing "Gender Identity Disorder" comes not from any natural cause but from cultural norms of stereotypical male and female identities. In fact, he argues, a feminine boy or a masculine girl is expressing an adaptive phenotype.
In the fifth edition of the DSM, published in 2013, the term "Gender Identity Disorder" has been replaced by "Gender Dysphoria," although the diagnostic criteria are mostly unchanged. People with "Gender Dysphoria" feel distress from "a marked incongruence between the gender they have been assigned to (usually at birth, referred to as natal gender) and their experienced/expressed gender" (DSM, 453). I assume that Poiani would object to this classification of feminine men or masculine women as showing a "mental disorder" rather than an adaptive phenotype.
To argue, as Poiani does, that the science of animal homosexuality shows that homosexuality is a naturally adaptive trait and thus not a "pathology" or "disorder" is a scientific argument with normative connotations. And yet, Poiani insists, science cannot claim to support normative conclusions without committing the naturalistic fallacy.
Natural law reasoning in general is often criticized for committing the naturalistic fallacy in assuming that a description of what is natural for us can support a prescription of what is good for us. But there is no such fallacy in natural law reasoning if we see it as reasoning through a hypothetical imperative.
Borrowing the language of Randy Barnett (1998), who has borrowed from the language of H. L. A. Hart about the "minimal content of natural law," we could say that all natural law reasoning depends on hypothetical imperatives that have a "given/if/then" structure: Given what we know about the nature of human beings and the world in which they live, if we want to pursue happiness while living in society with each other, then we ought to adopt a social structure that conforms to human nature in promoting human happiness in society. So, for example, given what we know about human vulnerability and human propensities to violent aggression, if we want to pursue happiness, peace, and prosperity in our society, then we ought to have laws against murder, rape, assault, and theft. Consequently, the laws against murder, rape, assault, and theft are natural laws.
Natural law reasoning does not prohibit us from punishing any expression of a natural behavioral propensity. For example, pure psychopaths are probably expressing their biologically natural propensities. But given what we know about the harmful propensities of psychopaths, if we want to protect our society from harm, then we ought to punish psychopaths to protect ourselves from their harmful behavior.
Consider how this would apply to homosexuality. Given what we know about the animal nature of homosexuality, if we want to pursue happiness, peace, and prosperity in societies with both heterosexual and homosexual individuals, then we ought to protect the liberty of homosexuals to live their lives as they wish, as long as they do not harm others. Consequently, the liberty of homosexuals would include the right to same-sex marriage, as long as we know that this is not harmful to others. That is the argument of Justice Kennedy's decision in Obergefell v. Hodges.
If one believes, however, as some of the opponents of same-sex marriage believe, that the governmental licensing of same-sex marriage will harm the children of same-sex parents, then natural law reasoning would condemn the legalization of same-sex marriage.
The debate here becomes an empirical question that will be settled by our reasoning about our experience: Is there any evidence that same-sex marriages harm children?
American Psychiatric Association. 2013. Diagnostic and Statistical Manual of Mental Disorders. Fifth edition. Washington, D.C.: American Psychiatric Publishing.
Bagemihl, Bruce. 1999. Biological Exuberance: Animal Homosexuality and Natural Diversity. New York: St. Martin's Press.
Barnett, Randy E. 1998. The Structure of Liberty: Justice and the Rule of Law. Oxford: Oxford University Press.
Poiani, Aldo. 2010. Animal Homosexuality: A Biosocial Perspective. Cambridge: Cambridge University Press.
Posts on related topics can be found here, here, here, here, here, here., here., here, and here.