I wish to take up again the subject of true friendship and marriage. I believe that true friendship and marriage are separate but equal institutions (to use a phrase with an unfortunate history). True friends are one soul in two bodies. Spouses are two souls in one body. A true friend is another self. A spouse is a selfed other united by body.
True friendship has always been rare, due to its requiring excellence of character and its being biologically unnecessary, but in our culture it is in an actual crisis, as in it is no longer valued or understood as it should be. Friendship is a term thrown around very casually and haphazardly in our culture, and most real friendships are friendships of utility or pleasure. While these lower forms of friendship are not necessarily bad, and even essential for our social cohesion as a species, they do not live up to the highest and most praiseworthy friendships venerated among the ancients.
One way in which I think many good people in our culture have tried to fix the crisis of true friendship is to try to appropriate it into marriage. True marriage is also in a crisis and that's another discussion which I will not get into here. I think that the two institutions require separate treatments, and while fixing one will help hold up the other, they cannot be addressed as if they were one without causing confusion which hinders both.
In order to be both true friends and a spouse with another, one would have to be both one body and one soul with them. This violates the fundamental uniqueness and individuality of each person, and so God has not made this possible for us. Marriage is the bond of the masculine soul and the feminine soul. The boundary in marriage distinguishes souls. In order to be good spouses, there has to be a bodily union of two different types of souls that complement each other. Marriage requires the maximal expression of masculine and feminine polarity; in fact the bodily union is impossible without it.
Friendship, on the other hand, requires that there be no carnal union. The boundary distinguishes the bodies. In the case of friendship between men and women, the masculine/feminine polarity must be minimized in order to maintain this boundary. In most cases true friendship is not plausible between men and women if there is a possibility of sexual attraction.
As I see it, then, in our very natures are written the propensity for man and woman to join bodies in marriage and man and man or woman and woman to join souls in true friendship.
I am speaking of marriage and true friendship in terms of their relation to sexuality in order to distinguish them, but sameness of sex is, of course, not all that true friendship is about, nor is marriage only about being complementary in sex. There are many aspects of the two that overlap, such as trust, endurance, fidelity, good-will, exclusivity, etc. I think the overlapping qualities of marriage and true friendship are part of the reason why people confuse the two.
It's helpful, then, to contrast marriage and true friendship. For instance, while both are exclusive, they are not exclusive in the same way or in the same degree. A true friendship is exclusive in the sense that a true friend is in a true friendship with some but not with others. On the other hand, the exclusivity of true friendship is not jealous or possessive as in marriage, because true friendship lacks an element of eros and, furthermore, does not constitute a union of bodies (as the body has a right to the possession and jealousy of itself, so does the union of bodies between man and woman in marriage).
The soul is more free to give and unite itself with a wider (though arguably not extensive) range of persons, for the soul is not bound by the same limitations as the body. Thus, while a body cannot unite with a third body without breaking union with the second, the soul can unite with more than one person simultaneously. Thus, marriage exhausts the unitive faculty of the body but true friendship does not exhaust the unitive faculties of the soul.
Another way in which true friendship and marriage contrast is in the direction of gaze. True friends, united in soul, gaze together at a common direction. Spouses, united in body, gaze inward at each other. So, for instance, true friends may grow together as men; they may gaze together toward the goal of being good men, but they do not give their manhood to each other or gaze toward the manhood of the other to possess it. Likewise, a man doesn't grow in manhood together with his wife; they do not share the common goal of excellent manhood; rather, he gives her the gift of the growth of his manhood, and she returns likewise with her womanhood.
These are just some of the reasons why I believe true friendship and marriage are separate but equal relationships. I do not mean in saying these things that men and women cannot be true friends with each other, but that they can only be true friends if the conditions are in place that make marriage between them impossible. And likewise, men and women can only be spouses if the conditions are in place that make true friendship between them impossible.
The mutual exclusivity of marriage and true friendship are not a bad thing at all; in fact by standing against each other they hold each other up. If it were even possible for true friends to be spouses, the world would be a worse place, because each person could "have it all" in one relationship, and every family might be tempted to become an island. But, by engineering us such that we cannot be one soul in two bodies with the same person with whom we are two souls in one body, God encourages us to turn also outside the family, and this is how He wants the love to spread around. And that makes the world a better place and fulfills us more as humans.