Frankly, the religious interpretation of marriage seems to do more harm than good in today's world. For example, before I got married I dated enough Catholic divorcees to know they are really messed up because of the divorce. They think they are lessened because of it, that somehow their purity and worth are diminished, because "marriage is made in heaven" and they failed at it. Now that is real-world consequences. If we instead realize that marriage is a form of social contract, and thus can be dissolved under the right circumstances, things would be better for everybody. Incidentally, it would also improve the marriage, because each spouse cherished a higher appreciation for the other, exactly because it is possible to leave. Conversely, if there is no such thing as exit, or the exit is particularly cumbersome, many people are then stuck in terrible relationships that make both spouses profoundly unhappy. So, let's cut the crap and de-sanctify marriage; it will make it better, not worse.
Marriage has always been a social contract. It was formalized legally to facilitate order in terms of the legatimacy of children, inheritance, property ownership, and so forth. Catholics have always separated marriage (the Sacrament of Matrimony) and the social contract (marriage license) and the basic marriage where papers are signed by both parties. Christians, especially Catholics, santified marriage as a contract between two people (man and woman for the purpose of pro-creation) and God (not the state). Diifferent religious faiths had different rules on dissolving the contract. 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 That may be a bit outdated, but it remains today.
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.
You may dissagree with the fundimental religious aspects, but they exist. Allowing for a civil union but not marriage would skirt the religous aspect and legitamize the legal aspects. Some would say this is outdated. Others feel that this change would fundamentally alter the purpose of a marriage and hence wrong.
It is nothing to me to have civil unions or homosexual marriages. I am no Bible toter, but I believe that the Bible is rooted in a belief that marriage is between a man and woman who could pro-create. There are passages in the Bible which I can't quote but I have a faint memory of them from the Old Testament.
From a religous perspective, the Roman Empire broke up because it became fundamentally corrupt in terms of its social fabric. The USA could do the same and it is headed that way. The government ties people of common goals together and when the common goals or fabric is broken up, the government fails. This is probably a bit simplistic since communistic governments remain today and governments controlled by Islamists remain strong (because the religion forces a very strict family structure with penalties as I understand it). Religious persecution goes back a long time and it continues today. 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.
Christianity is essentially a communistic and totalitarian system with redistribution of wealth as a basis for the good of everyone. But we know that it is not an effective system from a populace perspective due to human greed. Capitalism and individualism is not necessarily generic with a successful government, but allows for individuals to better themselves when they are not born on the "right side of the tracks" or within a certain family.
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.
But the country is faced with the breakup of the family unit which forms the basis of most Western civilization.
It is very complicated and one can not predict what will happen in the long term. But if the country makes the social contract as not significant, then you become more a vassel of the state rather than a member of a family.
Sorry for wandering a bit, but I will have to sort this out.