The Catholic Religion was probably the toughest at least to my knowledge. A divorce or breaking of the marriage contract had religious consequences... essentially dissolving the contract without the blessing of the Church resulted in being denied any of the "benefits" of the religions. That may be a bit outdated, but it remains today.
So, a bunch of people are stuck in a bad relationship, so if they stay, they suffer until dead. If they leave, their religion treats them like lepers. Then they try to move to new relationships and bring a ton of unresolved baggage, for all to enjoy. Outdated is the right word. But others come to mind as well. So, sanctified marriage does not seem to be a good idea.
Changing the social contract to be inclusive of same sex people changes the fundamental religious contract because marriage was for pro-creation from a religous perspective and a legal contract for the other aspects of marriage.
Except one person's religious convictions should be limited to him. It cannot be a free society if those convictions are foisted upon other people, either verbatim or by extension or by effect, especially those who do not share them. Just because in Bob's religion, marriage is between man and woman, does not give the right to Bob to tell Tracy and Kate THEY cannot be married because it is against HIS religion. If Bob tries to use the power of the state to affect what kind of contract the lesbos can get in, then Bob acts as a statist, theocrat, and tyrant.
From a religous perspective, the Roman Empire broke up because it became fundamentally corrupt in terms of its social fabric.
What exactly does that mean? What society does not have corruption?
Regarding the Roman Empire, this is the typical example given. But, closer examination of the historical truth shows the Romans were doing just fine for many centuries, and had several periods of tranquility and prosperity, usually when a competent emperor replaced a wastrel, or when the barbarians backed off for a while. It does not correlate at all with fall of morals. In fact, for most of its history, Rome was decidedly pagan, but it did fall just 150 years after Christianity became the official religion. Should we then conclude Christianity made Rome fall?
The reality is the Roman Empire example is the moral example given by religious leaders as a warning that loose morals destroy societies. Do as we say, or you will fall like Rome. No matter that it has nothing to do with the historical truth. But, it is a useful rationalization if it gets people to follow you. The method is not much different than telling the medieval people that pestilences are the way God punishes them for their sins, although the true cause is bad sanitation or a pandemic virus, both of which have nothing to do with the moral fibre.
Specifically regarding marriage, even in Republican Rome, it was considered a means of solidifying political and economic alliances, to be dissolved when the alliances were broken up as well. And yet, they did just fine for hundreds of years.
The big difference with America is that it was founded on Christian principles which are more or less generally accepted principles by other faiths. But the difference is that fundamental rights were God given and not granted by the government. The Second Amendment is one of those which many here hold dearly.
And the counterargument is the foundation was people willing to do violence to gain their freedom from an oppressive government. The religious aspect of it was necessary to counterweigh the opposite religious argument about viewing the king as the anointed representative of God on earth. Taking into account the revolutionaries had to gain the support of a lot of religious people, they had to provide the latter with a religious immunization against the religious memes of the loyalists.
How much changing the marriage contract would affect the government is debatable. But a country grows by population increase and same sex marriage is not compatable with that. As long as the number of homosexual is small relatively speaking, it probably has no significant effect on the US social fabric in itself.
How is gay marriage incompatible with population increase? if anything, it is neutral. if there are more gay couples adopting or using surrogacy, the effect would actually be positive on population growth.