Religious moralities do not come from gods. They come from human beings. They are assigned to gods as a way to give them an illusion of legitimacy - because history has shown that "You must obey me," tends to be far less effective than, "You must obey God, and I am the voice of God."
Religious moralities come from human beings, and human beings put into those moralities all of their prejudices and biases, their lust for power and to be served by others, their likes and dislikes and their arrogant presumption that they have figured out all of the truths to be known and any who disagree with them are not only wrong, but wicked, and must be punished.
One of the prejudices that nature has given human beings is the prejudice to view the objects of their own desires as having intrinsic merit - as being not only, "That which I like," but "That which deserves to be liked because of its intrinsic qualities." This leads to the attitude that "different is evil." When confronted with others whose likes are not compatible with one's own the tendency is to view those likes as "perversions" - as a warped and disgusting tendency to pursue that which has no or negative value.
"And I know it has no or negative value because I do not like it or actively dislike it."
The vast majority of us are heterosexual. Nature has created in us a strong desire for heterosexual acts. As I think back on my ancestors across millions and millions of generations I can tell you very little about a great many of them. Yet, I can say that, since the evolution of sex, none of them - or almost none of them - died a virgin.
Combine this with the unfortunate prejudice mentioned above and we see people reaching an unfortunate conclusion that heterosexual relationships are intrinsically good and tht there must be something defective in those who do not pursue that which intrinsically ought to be pursued.
This is an unfortunate prejudice in that it causes some people to do harm to others who do not deserve to be harmed. they do harm to others because it pleases them to see themselves as superior and "the others" as inferior. However, if there is any clear line in the moral universe, it is that doing harm to others merely because it makes us feel good sits on the far side of that line.
There may be a natural human disposition to view "different as evil" but it is a disposition that good people will try to hold in check. The institution of morality was invented for the purpose of establishing a set of institutions that hold our poorer nature in check and to allow our better natures to flourish (to promote desires that tend to fulfill other desires and inhibit desires that tend to thwart other desires). This means holding in check the disposition to view "different as evil" and to grant liberty to those who are different, so long as they are not dangerous.
Even here we must be wary, because the disposition to view different as evil - the good feelings generated by holding "us" as superior and "them" as inferior - will seduce us into seeing a threat where no threat exists. We WANT to believe that we have reason to do harm to those who are different, so we accept things as evidence even though they stand in bold conflict with reason and responsibility.
The Manhattan Declaration contains the following quote:
[Permitting homosexual marriage] would lock into place the false and destructive belief that marriage is all about romance and other adult satisfactions, and not, in any intrinsic way, about procreation and the unique character and value of acts and relationships whose meaning is shaped by their aptness for the generation, promotion and protection of life.
There is nothing in this but an expression of the attitude, "Different is evil." This is an expression of people who have taken what they like and assigned it extraordinary or supernatural virtue, so that they can generate in themselves the "good feeling" of seeing themselves as superior to those who are different.
This is bigotry and prejudice in its true form, prejudging those who are "like me" as superior, and prejudging those who "different" as "defective" and "worthy of condemnation."
It is accompanied by claims that "the others" are dangerous that make no sense when held up to the light of reason. They are claims of harm accepted by those aching for an excuse to act in ways harmful to others, and will grasp at any straw that can be offered as "justification" - to people who are similarly seduced by their own bigotries to accept.
They claim that marriage is meant for procreation and for raising a child.
Yet, nothing in a childless homosexual marriage is a threat to a heterosexual couple raising a child. And the belief that a homosexual couple must necessarily do a poorer job of raising a child has little or no basis in fact. People accept these claims, not because the research forces them to accept these claims, but because the pleasure of seeing themselves as superior to others seduces them into accepting these claims.
The most blatant irrationality with respect to the claim that these bigots have the best interests of the children at heart is the fact that many of those children are homosexuals. There is a very real harm being done to the interests of those children to put on the scale against the imaginary harms and benefits that spring from the bigot's imagination. It is a clear and obvious harm that is ignored because . . . well . . . "those who are different from us" are inferior and inferior interests do not need to be considered.
Even if there is a difference of opinion on this matter, morality demands that we give those who may be harmed the benefit of the doubt. The burden of proof is never on those who say that harm may not be inflicted, but always on those who say that harms must be inflicted. Furthermore, the moral person demands that the proof be something more substantial than, "It kinda feels like it might be a good idea." Harm requires proof beyond a reasonable doubt. The bigot's standards for evidence are much, much lower.
The authors of the Manhattan Declaration have made the situation worse. They have created a god in their image. They have assigned their prejudice to a god, creating a god that shares their prejudice and who, then, sanctions and defends the unjustified harms that the bigots would cause to "those who are different from us."
They pump up their chests with arrogant and false pride over the declaration that, "We have devoted ourselves to a higher power. We have devoted ourselves to pursuing God's will."
No, you have devoted yourselves to acting on natural prejudices and bigotries that you only imagine to be God's Will because that combines the good feeling of doing harm to those who are "different from us" to the good feeling of "serving a higher power who smiles on us when we do harm to those who are different from us."
When humans assign their prejudices and hatreds to a god, they then wrap those prejudices in an armor of faith where they can close their ears and refuse to listen to any argument that suggests that they are wrong. "God said that we may . . . indeed must . . . harm those who are different from us, and God cannot be mistaken."
Perhaps God cannot be mistaken. But humans can - and the gods that humans invent are prone to all of the same weaknesses and faults - prejudices and bigotries - of those who create their gods in their own image.