As for religion, not much else can be done there. Christian dogma will always be against same-s3x marriage.
So it is written. (1)
The law, however, can be changed - by no less than the people in a democracy. "Unacceptable" by the majority now, yes, but just like a lot of things were "unacceptable" back then.
I think the problem here is that some people base morality off what is deemed "acceptable" by the majority. If it is "unacceptable", it is wrong - which is a fallacy (
Argumentum ad populum - Wikipedia, the free encyclopedia). For example, slavery
is immoral because it infringes on the freedoms of those unlucky enough to be slaves, but a century ago it was acceptable. That is why acceptability, the consensus, and popularity absolutely have no bearing whatsoever in determining whether something is right or wrong.
So what is clear here is you need to understand that what the majority disagrees with, like gay marriage, doesn't make it automatically
wrong. You actually need to explain why it is immoral besides saying "because it is unacceptable". There are a variety of ways to do so, like saying it's unnatural, which in itself is also a fallacy (
Appeal to nature - Wikipedia, the free encyclopedia). So far, I've yet to come across a valid argument on why it is immoral.