In so many ways I tend towards this thinking however I believe that no one has the right to change what is considered a natural tendency that is marriage, something that has existed before government and before religion.
I'd say this is a very hasty argument to make. Both religion and marriage predate recorded history, and you couldn't say reliably that one came before the other;
almost as soon as hominids got to the proverbial scene, they had religious ideas. This isn't to say that marriage definitely came afterward; however, you couldn't possibly support the view that marriage came before government and religion, especially since religion is (understandably) one of the first things we bothered to do.
The challenge here, is what happens after its changed? Will those who fought to create this new "right" of marriage now be steadfast to hold to their definition when others come along to stake their claim at what marriage should be? For example what happens when polygomy is brought to the table? Will those who have redefined marriage not allow marriage to be redefined for these people? There are now rites that exist for those who wish to marry animals, which means this sort of practice exists already. Shall the definition of marriage be adjusted for them too? Once the camel's nose is the perverbal tent we have to expect the whole camel will eventually want to force its way in. Is society ready to make room for such adjustments now that it has opened the doors to same-sex marriage?
A slippery slope argument, you say? How wonderful!
Here's the rub: we "secularists", to the extent that that is in fact a group of people who share similar viewpoints, are actually logical human beings. There is, in fact, no secularist agenda to speak of; what we want is for everybody to have equal rights, regardless of any competing religious viewpoints.
What we have found is that marriage offers people certain economic perks; as is well-documented in this thread, you're better off economically if you're married. This is fine, except that there's an entire group of people that can't get married by definition - homosexuals. By the very nature of marriage, this right is systematically denied to homosexuals who want to get married, and we "secularists" think that is gross.
How will we keep from re-redefining marriage in the future? Namely because there's no logical reason to push for anything else. As soon as you can marry someone of your own gender, things are equal; marriage is now defined as
any consenting person forming a loving union with any other consenting person, which is perfect because the right of marriage is denied to nobody. Polygamy or animal-human marriage don't come with the package because this definition is fine, and because polygamy and zoophilia have always been illegal in this country anyway. Even "secularists" concede that there's no reason to redefine marriage to include these things, as there's no logical or social support for them.
Also, don't be scared of re-definition. Contrary to your belief, marriage is not some unchanging, eternal fact of life. In fact, I recall
a pretty significant change made to marriage not long ago which is, in my opinion, analogous to the situation we're in now. And people didn't start having sex with goats once this decision was made, mind.
Right now we are seeing a growing majority of secularist authority.
Again, this is incorrect. This is an overwhelmingly religious country, and our "authority" -- congress, executive branch, etc. -- is overwhelmingly more Christian than the country as a whole. Perhaps what you're noticing is a growing acceptance of nonreligious ideas, but I'd argue that it's a good thing when people start to care more about equality than what they think their god might have to say on the topic.