Among the biggest problems with LGBT marriage is that it destroys a core social institution. If you destabilize something as central as marriage by diluting it into an amorphous liquid category that's nearly totally divorced from the reproduction of future generations then you end up killing the institution by over expanding it.Society is stabilized by certain relationship categories that clarify people's obligations to each other, and if you dilute them then confusion sets in and people don't know how to relate to each other. This can happen with authority structures. If no one is "the boss" then role confusion ensues because everyone is either competing to act like the boss or not knowing who to take orders from (this is among the biggest problems with democracy, it's rarely sustainable because it collapses into a competition for clear and certain power). If marriage becomes something that society can simply redefine the meaning of, then it ceases to exist as a "real" predictable thing and becomes a mere mental construct open to interpretation.This is where religion comes in. The Christian Bible defines marriage and thus renders Western marriage "real." God created it, so it's eternally real. It’s a sacred immortal fact.Society might say, "we want to change the meaning of this," but if they do that then they remove it from the category of eternal truth, and it becomes a trivial thing that has no permanent meaning. The role association system that we have, the idea of commitment associated with love sealed by the institution of marriage, suddenly becomes unstable, and we lose the ability to see it as meaningful if it’s expanded into all possible human combinations.So, it's not that LGBT marriage itself is the original threat, its merely another threat working to destabilize society's institutions. Premarital sex, living together outside marriage, adultery, and divorce all produce the same result of marriage being redefined or corrupted into something that's either irrelevant or unworkable. Because human society and interaction is based on symbolic understandings, like a man not flirting with a girl because she's wearing a wedding right, then when those symbolic understandings disintegrate communication is undermined and society begins to disintegrate. People lose trust in each other because they're anxious about life’s new uncertainties. People end up wandering through life wondering what's going to change next (" liquid modernity "). Nothing is worth investing in, even human relationships.You might say that society has changed and we need to create a new way to operate that includes LGBT marriage, but the question still remains: Where is the god who will make our new norms "real" instead of just the product of people's recent whims? Where’s the authority to ensure that the new order of human communication about marriage is accepted? That new form of transcendent authority hasn't emerged, and until it emerges people still have good reason to believe that marriage is still just between a man and woman. You might not believe in God, but it's still more efficient to believe in a dead god's interpretation of marriage than it is to believe some random new idea about marriage that emerged in the 2000s because a bunch of people decided it was a good idea.