I know, I know, these questions sound abstract. But consider the claim that "all men are created equal." Does this claim mean that all male human persons are created equal to one another because they all have a male human nature? But this understanding of the claim would exclude female human persons, presumably because they do not have a male human nature. Thus the claim that "all men are created equal" should be understood to mean that all human persons have a human nature, which is the basis for human equality.
However, when I was an undergraduate in the 1960s, then-fashionable existentialist thought were fond of saying that there is no such thing as human nature. At that time, anyone who thought that there is such a thing as human nature would not be considered to be fashionable in existentialist intellectual circles at that time.
Post-modernist thought is now more fashionable in academia than existentialist thought. However, like existentialist thought, post-modernist thought tends to eschew the use of the conceptual construct of human nature.
This post-modernist tendency is so pronounced today in academia that Martha C. Nussbaum published an article titled "Human Functioning and Social Justice: In Defense of Aristotelian Essentialism" in the journal POLITICAL THEORY, volume 20, number 2 (May 1992): pages 202-246. As she indicates in her subtitle, Aristotle is usually credited with working with the conceptual construct of the essences of things. For example, the essence of human persons is human nature. As she indicates, if we were to be banned from working with the essences of things such as human nature, we would be severely limited in any argumentation that we might want to advance regarding social justice. In other words, if you want to argue that there are universal human rights, then you should base your argument on the fact that human nature is universal.
Digression. I would like to point out that the way in which Nussbaum goes about defending Aristotelian essentialism involves what she considers to be human functioning. But another Aristotelian philosopher named Bernard Lonergan focuses on a different view of human functioning, namely cognitive capacities and operations, in his masterwork INSIGHT: A STUDY OF HUMAN UNDERSTANDING (1957). Lonergan's follower Frederick E. Crowe spells out the full implication of Lonergan's thought for our understanding of human nature in his fine 1965 essay "Neither Jew nor Greek, but One Human Nature and Operation in all," which is reprinted, slightly revised, in COMMUNICATION AND LONERGAN: COMMON GROUND FOR FORGING THE NEW AGE, edited by me and Paul A. Soukup (Sheed & Ward, 1993, pages 89-107). End of digression.
Now, many Americans today see the issue of same-sex marriage as a social justice issue. If men and women both have a human nature, why should it make any difference in civil law if marriage is operationally defined as being between one man and one woman, or as being between one man and another man, or as being between one woman and another woman?
However, during his annual Christmas address on December 21st, Pope Benedict XVI argued that civil marriage should be operationally defined as being between one man and one woman.