Through the voice of the playwright Aristophanes, Plato in his Symposium tells of a wonderful myth regarding the meaning of love. In the beginning, says Aristophanes, humans were comprised of having two halves: male halves and female halves (hence having four arms, four legs and two heads). Due to the authority of Zeus, he punishes the human race for misconduct and splits everyone into two pieces. Since then, wanderers are walking the earth in search of their other half. In this story, we see (perhaps not so much an explanation of homosexuality) but a placement of homosexuals and heterosexual on a similar plane of sexuality. As Aristophanes writes:
And so, when a person meets the half that is his very own, whatever his orientation, whether its to young men or not, then something wonderful happens: the two are struck from their senses by love, by a sense of belonging to one another, and by desire, and they don’t want to be separated from one another, not even for a moment. 
While this mythic account of the meaning of love may contain some kind of aesthetic appeal to us, the issue isn’t of course that simple. There is something more grander taking place in the mutual relationship of two lovers: namely, a striving for “the good,” where the well-being and the self-realization of each partner are of overriding importance to one another. As Karol Wojtyla (a.k.a. Pope John Paul II) argues in his book, Love and Responsibility (1981), I as a person desire the good for myself. In “loving” another person I am not using them as a means to my own central end. Rather,
I may want another person to desire the same good which I myself desire. Obviously, the other must know this end of mine, recognize it as a good, and adopt it. If this happens, a special bond is established between me and this other person: the bond of a common good and of a common aim. This special bond does not mean merely that we both seek a common good, it also unites the persons involved internally, and so constitutes the essential core round which any love must grow. 
The significance of this point – “two individuals who consciously choose a common aim” (p. 28) – is so that both individuals are on the similar footing of equality and it “precludes the possibility that one of them might be subordinated to the other” . In this post I want to elaborate on this line of reasoning within the context of the same-sex marriage conversation and how the essential differences between man and woman are illuminated when we have a definitional focus on traditional marriage.
Human actions are ordered to an end. In other words, we act insofar as we have a reason or purpose for doing that action. This is predicated on the Aristotelian view where the “good” (agathon in the Greek) is defined as “that for the sake of which everything else is done. . . ”  In other words, Aristotle believed that different things are called good “not because they all contribute to one end, but because they all contribute to their respective ends” . As Ralph McInerny notes, Aristotle did not restrict himself to the good as only applying to these respective ends. There is furthermore, “an overarching, comprehensive, ultimate end of all that human beings do” .
To finish our brief section here, consider the following example: suppose you were to intend a certain end – say, climb a mountain. With that ultimate end in mind, you then clarify steps to achieve that end – bring supplies, food, tents, etc. The point to stress are those steps that you take – i.e., you function off of a rationale that has the ultimate end in view. Whenever you have an end such as climbing a mountain in mind, you never abandon that focus while you are creating steps towards reaching that end.
The Utilitarian vs. Personalistic View
We can distinguish between what we may call “the utilitarian view” and the “personalistic view.” In the first view, we understand that “Man in his various activities makes use of the whole created universe, takes advantage of all its resources for ends which he sets himself, for he alone understands them” . With respect to the economic life, natural resources and so forth, using things for our own personal end as human beings is certainly fitting. As John Paul writes, “Intelligent human beings are only required not destroy or squander these natural resources, but to use them with restraint, so as not to impede the development of man. . . [but] to ensure the coexistence of human societies in justice and harmony” .
One question we should ask is how this whole thing changes once we apply them other human beings. Is it right to regard a person as a means to an end? The usage of a person as merely a means is precluded by the fact that they are a person: one who is a thinking subject, can make decisions, and can create natural aims of his/her own. This fundamental understanding of personhood is what John Paul calls “an inherent component of the natural order” . In the second view, the personalistic, love is understood as the opposite of using.
As argued above, where my end to seek the good is adopted by another individual and hence, we both now consciously seek a common good, we may understand love as something exclusively portioned for human persons. As John Paul writes:
When we look at man, we discern in him an elemental need of the good, a natural drive and striving towards it. This does not necessarily mean that he is capable of loving. In animals we observe the manifestations of an instinct which is similarly directed. But instinct alone does not necessarily imply the ability to love. This capacity is, however, inherent in human beings and is bound up with their freedom of will. Man’s capacity for love depends on his willingness consciously to seek a good together with others, and to subordinate himself to that good. 
With this in context of a sexual ethic, i.e., moving to an environment where this involves man and woman, this principle is put into practice with marriage. For, in marriage, a man and woman unite in such a way that they become “one flesh” (to use a phrase from Genesis 2:24). Consider what John Paul has said in his Theology of the Body (November, 1979) regarding this unification of man and woman:
In this way, we find ourselves almost at the heart of the anthropological reality that has the name “body.” The words of Genesis 2:23 speak of it directly and for the first time in the following terms: “bone of my bones and flesh of my flesh.” The man uttered these words, as if it were only at the sight of the woman that he was able to identify and call by name what makes them visibly similar to each other, and at the same time what manifests humanity. 
This unity of man and woman (i.e., marriage) has such an end concerned with procreation, the family unit, the building of a future generation, and at the same time the continual flourishing and ripening of a relationship between two individuals, “in all the areas of activity which conjugal life includes” .
Man and Woman
To borrow a helpful understanding from Robert Anderson, Sherif Girgis and Robert George , marriage is a comprehensive union of persons. As mentioned earlier, this borrows from the personalistic view in that it supposes that your body is an essential part of you; i.e., according to their book, “Since the body is part of the human person, there is a difference in kind between vandalism and violation; between destruction of property and mutilation of bodies” . Hence, this is why rape and even torture is peculiarly wrong due to mistreatment (or abuse) of the person and not merely a “disruption of property,” (the body) like someone vandalizing a private residence.
What kind of union are we talking about? For one, it is a union where two individuals engage each other by consent to achieve certain goods for common ends. This is how we understand marriage to be comprehensive, by “unifying activity, unifying goods, and unifying commitment” . These two individuals are unified in mind and body, so that their wills are both unfolded to reveal shared lives and shared resources. Discussing more on this subject of unity, consider what Girgis, Anderson and George explain at length:
[M]arriage unites [man and woman] in pursuit of every basic kind of good. In particular, marriage calls for the wide-ranging cooperation of a shared domestic life, for it is uniquely ordered to having and rearing children: The comprehensive good of family life enriches a marriage as such, and lack of children is a lack for a marriage, in a way that is not true for best friends, roommates, or teammates. 
This point is significant because it identifies man and woman as having socially and biologically distinct features as mother and father, both with essential functions for protection, sanctity, love, and etc. for each other and the children that these two might create. According to Greg L. Stanton and Bill Maier’s book , there are a couple reasons for supposing this:
- (1) “Marriage always brings male and female adults together into committed sexual and domestic relationships in order to regulate sexuality and provide for the needs of daily life. Wives help men channel their sexual energy in socially productive nonpredatory ways. Husbands help protect women from the exploitation of other males.
- (2) “Marriage ensures that children have the benefits of both their mother and their father, each in their distinctive and unique ways” .
-  Quoted from Andrew Sullivan, Same-Sex Marriage: Pro and Con (Vintage: 2004), pp. 5-6.
-  Karol Wojtyla, Love and Responsibility (Ignatius Press: 1981), p. 28.
-  Ibid., p. 29
-  Jack Wheeler, “Rand and Aristotle: A Comparison of Objectivist and Aristotelian Ethics,” The Philosophic Thought of Ayn Rand. ed. Douglas Den Uyl and Douglas Rasmussen (Illini Books: 1984), p. 83.
-  J. A. Stewart, Notes on the Nicomachean Ethics of Aristotle (Oxford University Press: 1892), p. 88 – emphasis added.
-  Ralph McInerny, The Cambridge Companion to Aquinas, ed. Eleonore Stump and Norman Kretzmann (1997), p. 198.
-  Wojtyla (1981), p. 25.
-  Ibid., p. 25.
-  Ibid., p. 27.
-  Ibid., p. 29
-  Pope John Paul II, Theology of the Body (The Catholic Primer: 2006), p. 25
-  Wojtyla (1981), p. 30.
-  Robert Anderson, Sherif Girgis, Robert George, What is Marriage? Man and Woman: A Defense (Encounter Books: 2012)
-  Ibid., p. 24
-  Ibid., p. 23
-  Ibid., p. 28
-  Gregory L. Stanton and Bill Maier, Marriage on Trial: The Case Against Same-Sex Marriage and Parenting (IVP Academic: 2004)
-  Ibid., p. 22.