Let's get back to the question of justice, which has been the consistent foundation of the arguments offered by those who are advocating that the traditional understanding of marriage be stretched into something that includes same-sex couples. (I understand that, in Episcopal Church politics on the eve of General Convention, there is more in play here than gay marriage, but--let's face it--it all boils down to that. Whether we're talking about actual marriage or marriage-like relationships being liturgically blessed and recognized as sacramental in nature, the premise is the same, and the matter of partnered lesbians and gays being consecrated to the episcopate then becomes simply a corrollary.) The talking point is something like this: Marriage is a sacramental relationship between two people, a relationship that participates in and is a sign of the mystery of God's redemptive love. So it is fundamentally unjust to extend the privileges of marriage to heterosexual couples and not also do so to homosexual couples. It amounts to invidious discrimination. Anyone who is baptized is potentially a candidate for any other sacrament.
Taken on its face, there is a certain inexorable logic to this line of reasoning. But the quality of that logic is only as good as the premise upon which it is founded. Is the "premise of the question" worthy of acceptance? Let's take a look. Here's what I see as the premise underlying the "justice" argument: Human beings are instinctively inclined to form (what biologists might call) "pair bonds." For the majority of people, the field of potential mates toward whom this inclination is configured is limited to members of the opposite sex. For a minority, it is limited to members of the same sex, and for a still smaller minority, it can be someone from either sex. (Yes, I know I'm not accounting for those who consider themselves "transgendered," but I'm not writing a book here.) Thus, an entire class of people--that is, those whose ability to form pair bonds is oriented toward members of their same sex--is thereby discriminated against by being denied the same opportunity for liturgical blessing and sacramental recognition (which includes such homey privileges as coming up for an anniversary blessing during the Sunday Eucharist) that is accorded to heterosexual couples.
However, this premise rests on a shaky foundation. It slices off and isolates one facet of what marriage is--a relationship of mutual companionship--and invests it with a life of its own, decontextualized from the whole institution of marriage. It neglects the Church's theological conviction that marriage was "instituted by God in creation" (BCP, p. 423, emphasis added); marriage is no mere human social construct that we are free to tamper with as seems expedient, but is part of the given fabric of who we are as human beings. Moreover, it is inherent to the given character of marriage to be ordered by the gender polarity (what some have called sexual dimorphism) that is also "from the beginning"--i.e. "male and female created he them" (Genesis 1:27). The fact the people (with the rarest of possible exceptions) come in two varieties--male and female--is not an insignificant datum of biology, but rather a highly significant datum of Divine design.
Then there is what we might call the "cosmic" dimension of marriage, laid out for us in Ephesians 5 (21-33) and referenced conspicuously in the aforementioned preface to the Prayer Book marriage rite. Christ (ascended and glorified but still very much the "man" Jesus) is the bridegroom and the Church is his ... wait for it ... bride--not generic partner or companion, but bride.
Perhaps the most obvious reason for not uncritically granting the premise that any pair-bond between non-related mutually consenting adults is an appropriate candidate for the institution and sacrament of marriage is what is arguably still the principal social end of marriage, which is to provide a stable environment for a pair-bonded couple to raise the biological offspring of their relationship to adulthood with as few risks as possible. Individual exceptions notwithstanding (heroic single moms and single dads, devoted adoptive same-sex parents, dysfunctional and/or abusive two-parent couples raising their own kids), secular society knows, and the Church knows, that the traditional norm is still the best thing we've got going for turning out well-brought up adults. It deserves to be privileged. We all benefit from the good parenting done by couples who bring up the children they brought into the world.
So if the premise behind the "justice" argument cannot be sustained, then the ensuing logic also falls apart. Yes, it is discriminatory to extend the privileges and responsibilities of marriage only to opposite-sex couples. But such discrimination is neither arbitrary nor invidious. Rather, it is rational, and well-founded. Therefore, it is not unjust. In moral theology, justice is about giving someone what is legitimately due them. When I make a payment on my auto loan each month, I am "doing justice," because that amount of money is legitimately due the company that made the loan. Continuing to reserve the institution and sacrament of marriage to opposite-sex couples is also "doing justice," because such preferential treatment is what is "legitimately due them" in the light of the entire ambit of the Christian tradition--scriptural, theological, and poetic/mystical. And this is neither insult nor injury--i.e. not unjust--to other sorts of pair-bond relationships because marriage is not something that is legitimately due such relationships. If I were to decide, for whatever reasons, that I wanted to become a citizen of, say, Poland (after all, I do live in Warsaw!), the Polish government would not be abrogating justice, but, in fact, serving justice by denying my request for citizenship. Since I don't know that I carry a drop of Polish blood, nor do I know anything of Polish language or culture beyond the fact that I like their sausage, there would be good rational reasons for doing so. Marriage is not an abstraction that can shape-shift to suit our desires. It is a concrete reality with its own inherent character. Bending the definition of marriage to include same-sex couples would make it something else entirely. The demands of justice require us not to do so.