The Manhattan Declaration is a declaration written to present a particular religious view on moral issues regarding life, marriage, and religious liberty. The Declaration is so filled with holes and inconsistencies that they easily identify the document as being, not the work (or the word) of some divine wisdom, but the flawed work of mortals - and bigoted, arrogant, self-serving morals at that.
They are the words of people who recognize - either consciously or unconsciously that, "Do as I command," tends to be far less effective then, "Do as God commands and I am the word of God," and so they give in to a craving to assign their bigotries and prejudices to God in a document that they called the Manhattan Declaration.
I have identified a number of flaws already - flaws that a fair and humane person would have recognized, but the authors, blinded by their bigotries, their arrogance, and their self-serving desires made themselves did not recognize.
Another of the flaws that no fair and just God would endorse, but which fills the needs of a bigoted human seeking to inflict harms on the interests of others and coat it in an illusion of legitimacy, is the claim that if we permit homosexual marriages we must also permit heterosexual marriages.
They make these argument in spite of the fact that heterosexual incestuous marriage has none of the flaws that the authors suggest are reasons to condemn homosexual marriage. Heterosexual incestuous couples can have children and, in having children, have an interest in seeing the marriage protected so that both parents can serve in raising that child. The child would have a mother and a father. As such, all of the arguments these bigots offer for protecting the institution of marriage are arguments for protecting and preserving heterosexual incestuous marriage.
If there is a reason to reject heterosexual incestuous marriage, then it is independent of the reasons for rejecting homosexual marriage. This places the bigots who use this argument in a logical bind.
If there is no independent reason to justify condemning heterosexual incestuous marriage, then these bigots themselves have provided the justification for recognizing these marriages as proper and legitimate marriages within their definition.
If, on the other hand, an independent reason to condemn heterosexual incestuous marriage exists, then it is not the case that permitting homosexual marriage implies permitting heterosexual incestuous marriage - because heterosexual incestuous marriage would still run afowl of this "independent reason."
The same dilemma applies to plural marriages. Plural messages, likewise, can result in children and for the same of whom the marriage should be protected and preserved. In fact, many of the arguments these bigots give for the value of heterosexual marriage are even more true of a polygamous marriage. The death of a single parent or an accident causing permanent injury, the loss of a job, or severe illness is far more devastating to a two-parent family than to a multi-parent family. So, these bigots themselves provide us with reason to recognize and protect these marriages.
If there is any independent reason to reject these marriages it is not in virtue of what they have in common with homosexual marriages (because they have far more in common with heterosexual marriages), but must be some other reason. This "other reason" buts the bigot in the same bind that they are in with incestuous marriage.
A fair and just person would see these flaws - or at least be able to show that he put a responsible effort into finding them. Any god created by such a person would also be fair and just - a god that also condemned the bigoted, arrogant, self-serving people who irresponsibly blinded themselves to these moral problems.
However, the authors of the Manhattan Doctrine have shown themselves to be too deeply blinded by their bigotry to examine their own arguments for signs of unfairness and injustice. They are not looking to do what is right. They are looking to give their own bigoted, arrogant, cruel, unjust sentiments an illusion of legitimacy. They do this by inventing a God that is just as bigoted, arrogant, cruel, and unjust as they are and then declaring, "These are not my actions. These are commanded by God" - a god the bigot has created in his own image.
As it turns out, there are, in fact, independent reasons to justify condemning heterosexual incestuous marriage (and, by extension, other forms of incestuous marriage). These reasons have nothing to do with the problems of genetic illness. It would scarcely be considered just to nullify a marriage on the grounds of the genetic inferiority of any children that may result.
The argument against incestuous marriage is grounded on the real-world observation of the great deal of harm and suffering that results from incestuous abuse generally. The only way we could permit heterosexual incestuous marriage is if we were to lower the social barriers against incestuous relationships generally. Lowering the psychological barriers against these types of relationships runs the very real risk of causing a significant increase in the amount of overall incestuious sexual abuse. People will act to fulfill the most and strongest of their desires (given their beliefs). A weaker aversion to incestuous relationships can not help but result in an increase in incestuous acts.
So, the many and strong reasons we have for reducing these types of abuse and the harms they cause are many and strong reasons for promoting an overall aversion to incestuous relationships. It is an aversion fed somewhat by a natural disinclination towards incestuous relationships. However, nature's inclination is clearly not as strong as it should be - given the number of events that still occur, and the harms that could be prevented if the aversion were stronger. Thus, morality calls for socially strengthening (though its moral institution) this (amoral) natural aversion.
The moral institutions for strengthening that aversion catches incestuous marriage in its net. An inversion to incestuous relationships necessarily implies an aversion to incestuous marriage. The result is a moral prohibition on such marriages.
However, this moral prohibition says nothing about the morality of homosexual marriages (other than to say that homosexual incestuous marriage should also be prohibited - a conclusion argument that the 'genetic immorality of incest' theory cannot handle).
This argument will not appease the bigoted, arrogant, self-serving authors of the Manhattan Declaration. It will not justify the harms that they seek to inflict on others. They want an argument that will give an illusion of legitimacy to their own behavior, motivated as it is by their own unjust and unkind prejudices. The clearly flawed argument that they wrote into the Manhattan Declaration serves that purpose – as long as they blind themselves to its flaws. But blinding oneself to reason is something that bigots have historically shown themselves to be quite good at.