Like a good Kantian, I have tried to focus on the moral reasons for same-sex marriage in my attempts to conduct a reasoned discourse on the public record. Here I will change tracks and look at the reasons why the traditional prohibition of same-gender relationships is not viable irrespective of the witness of Scripture and Tradition on the subject. The following constitutes the bare minimum I expect answered from anyone who wishes to maintain the traditional view, failing which I will take the point to be conceded and expect unanimous assent next General Synod. Apologies for my absence; coverage of Divnity orientation week will follow shortly.
It assumes the distinction it seeks to create.
Ultimately, the fatal flaws of the reasserter argument are its question begging and special pleading. Proponents point to a handful of texts concerning some form(s) or other of ancient near eastern same-sex activity. At the same time, the Church blesses heterosexual marriages, about which St Paul is hardly more enthusiastic. Thus in electing not to do the same for same-gender couples, the Church tacitly assumes that there is something qualitatively different about them from the relationships that it does bless. As this is ultimately the conclusion of the reasserter argument, it cannot also be a premise, much less a "missing" implied one. Leaving aside the biblical texts (horrors!), the problem is the paradox of our own practice. Apart from a nod to procreation - and even that is paired with the Godly raising of children - there is nothing in our marriage service that limits its scope to heterosexuals. (I noted with interest that over at the Anglican Samizdat there was as much outrage over +Montreal's tweaking of the existing blessing of civil marriage rite as +Niagara's publication of a rather twee de novo one, even though the former could hardly be said to change any doctrine since the wording is virtually identical apart from the pronouns).
If a distinction is to be made in the Church's response to heterosexual and homosexual relationships, it must either be rooted in some distinction inherent to the relationships themselves, or else an arbitrary commandment or "chuk." Reasserters, unsurprisingly, have not been able to account for this distinction (hint: it's not there).
It has decidedly questionable implications for our sacramental theology and our theological anthropology of gender.
Since the only distinction from all heterosexual unions common to all same-gender unions is the gender part, the assumed qualitative distinction, if it exists, must subsist therein. This model then has to elevate a biological triviality to the level of dogma. If we are going to accept this position, as reasserters would have us do, we should have to admit that genitalia are or can be a real barrier to receiving a sacrament - leaving the "I've got mine" female Essentials clergy who exhausted their capacity for "innovation" in 1976 on thin ice. Reasserters are asking us to accept a view that Anglicans (including most of themselves) rejected with respect to ordination. To arrive at a contrary and inconsistent view with respect to marriage belies their claim not to be singling gays out. It's all very well to complain about revisionist teaching, but rather more opaque when one wishes to pick and choose the instances in which revision is applicable once it has already been made. (As they say in the US Congress, wasteful spending always seems to be done outside one's own district).
It creates a class of Christians who have no hope of fulfilling the Church's teaching on chastity
While heterosexual Christians have the option of marriage, gay Christians are placed in a position whereby anything they do is automatically categorized as sexual sin. Apart from the relatively few Christians of any sexual orientation with a genuine celibate vocation, it is never quite clear what those who find themselves to be gay are meant to do under the reasserter model. Once they have made their case for why we are SOL, they are alarmingly light on advice.
Its credibility is undermined by its proponents
Apart from the odd kapo à la David Ould, the antigay industry is largely a unidirectional enterprise engaged by heterosexuals against a group in which they can be assured of never finding themselves. It is prima facie suspiciously convenient that the form of relationship uniquely sanctioned by God in their view is their own. As Anne Lamott says, "You can safely assume that you've created God in your own image when it turns out that God hates all the same people you do." Reasserters will take pains to profess Christian charity, yet it is still hard to fathom why even someone sincerely convinced of the traditional position on homosexuality would make a theological career out of seeking to deny a sacrament to other Christians. In the secular world, the slogan is "Don't like gay marriage? Don't have one." It is one thing for groups like Essentials to be averse to the notion of equal marriage, but to insist that one simply cannot inhabit a communion where two blokes might tie the knot in the next parish over is decidedly odd from the standpoint of Anglican polity.
It places the burden of proof on the accused, and holds them guilty until proven innocent.
Reasserters avoid defending their position in detail by maintaining that the onus is on those who seek change to prove their view beyond a reasonable doubt. This is backwards, as gays are the ones in the dock and in any case reasserters typically accept Anglican "innovations" on contraception, and yet wish to revert to a strictly procreative model when confronted with a different behaviour they happen to find less palatable. The onus is on reasserters to account for the disparity in their positions if it is to be taken as anything beyond a subjective and rather irrational statement of personal preference.
Certain aspects of it contradict known empirical data
Reasserters typically point out that questions about the aetiology of homosexuality are not settled. A biological or genetic basis has not been proven, they say. The first problem with this, as above, is that it assumes the distinction and is really a question mal posée. Most would be bemused if questioned about the inception of their own sexual orientation, but gay people are assumed to be different. Insofar as we do not ask what "causes" heterosexuality, a corollary question about homosexuality is unintelligible. Whatever the cause of sexual orientation should prove to be, there is no reason to believe that it is different for different sexual orientations. Many however harbour a lingering assumption that heterosexuality is the default or natural human state of origin, from which a few simply fall away. This is easily verifiable: ask a gay person if they previously experienced predominate attraction to the opposite sex. In most cases the reply will be negative, and the fact that sexual orientation sometimes shifts of its own accord over the lifespan doesn't mean that it can be planned on.
The second problem with the above is that it misses what is relevant in the moral debate. Whether homo- and heterosexuality is fixed in the womb, the nursery school, or immediately prior to puberty is something scientists will continue to consider throughout this century - no doubt the answer(s) will be multifaceted. At this stage, however, we do know that it is fixed early on, usually immutable, and almost always involuntary, and that there is no difference between sexual orientations in this respect. Thus while reasserters may claim that the opposite-gender marriage definition is not like anti-miscegenation laws because "being gay is not like being black" the fact is that we do not eschew racial discrimination because race is a genetic trait (insofar as it is a real trait at all) but because of its involuntary and immutable nature. Now I'm not a lawyer but you don't need to take my word for it. In explaining what constitutes an unlawful ground of discrimination, the Supreme Court of Canada speaks of traits that are "immutable or changeable only at unacceptable cost to personal identity." Thus a trait like sexual orientation is held to be "analogous" to race or religion in the sense relevant to us. That some such traits are genetic and others are not does not affect our position.
Moreover, the anthropological assumptions of the argument fall here as well. The Bible indeed says "male and female he created them" and yet we now know of individuals who do not fit neatly into this binary - even (leaving aside questions like transsexuality) in its narrowest understanding, since there are cases of patients whose chromosomal configuration is neither XX nor XY. (If we do not accept such a reductionist view - which as presumably incarnational-dualist Christians we ought not - we have even less grounds for holding the traditional position).
It appears entirely unconcerned with these questions.
Reasserters can be quite casual in assuming that their position is correct and uniquely "orthodox", and it's easy to be complacent when yours is the only position for several centuries. Yet they are remarkably silent when it comes to overcoming these hurdles to accepting their position. As the "reappraiser" argument is distilled and refined, reasserter discourse adds nothing new, preferring to repeat its stump speech about Creation, Paul, and dubious Thomistic metaphysics. Conversations like this show the reasserter tendency to fall back on an ipse dixit when confronted with the hard questions about their position's less attractive implications. They can't answer the objections, and yet do not concede them, an untenable position in debate which more or less boils down to believing gays are sinners because one wants to. This is not surprising to those of us who recognize it as the only grounds, however specious, on which the position can be held, but reasserters continue to demand deference to their view disproportional to the evidence they have for it. It's natural to instinctively dislike something different from us, but if we wish the Church to take us seriously as an "orthodox" voice and repent its ways, we must have stronger reasons than "because I say so." The heterosexual male revulsion at same-gender sex (as they imagine it to be) is understandable psychologically but it is not an argument and until it is backed up by one the Anglican Church of Canada should not be expected to dignify it.