Sure. Marriage is the union of a man and woman in holy matrimony; a proclivity for any particular perversion or fetish is a pathetic excuse for undoing an institution that has been the basis for the family unit and civil society for Western nations for years. I'm sure a cursory internet search would reveal dozens of fetishes and perversions which people have never heard of and we've truly sunken into nihilism we conflate these with traditional marriage.
1. "It promotes the abnormal order."This is actually true; while homosexuality may be natural it is an abnormality in humans (thus it's rarity) so I guess they do have that in their favor.
2. "It promotes different cultural norms." This is also true; while it's questionable if it's really admissible (appeal to tradition) it does indeed change the social order and that is a valid grounds to allow or disallow something because half if not more than half of the things really allowed and disallowed revolves around emotional and psychological comfort than anything else. Pragmatism, despite our "lofty seat" over all the other animicules, really plays less of a role in our lives than we think.
3. "Marriage is a privilege not a right", actually sadly this too is true. Marriages are not required to be honored if the justices deem it not valid or otherwise unreasonable. We can see this with immigration issues where you can't just "marry up" the moment you cross whatever border. So because of it's status as a privilege people argue that they can bar against certain entities and that argument is not actually faulty. It's weaker but it's not faulty.
So there are three examples circulating.
One thing leads to another. You allow gay marriage then what will people ask for next? You think there's a stopping point to how people act? It's true in the future our human world will be different but I don't want it to change so suddenly. And you think it won't happen? Check how alcohol was illegal, how it became legal and then now people are asking to legalize weed.
To establish a same-sex union or marriage substantiates a normality to the practice. In other words, it makes the practice of same sex an openly acceptable lifestyle in society.
There's a unique problem with this, however, when you look at the statistics of countries who have already allowed same-sex marriage. Initially, the number of same-sex marriages spiked. However, the number has substantially dropped. Data compiled shows that approximately 96 percent of homosexuals don't get married when they are given the opportunity. And those that do get married break up at a much higher rate than heterosexuals. Michelangelo Signorile, a leading advocate for the homosexual community has admitted that his goal isn't to get government-backed same-sex marriage so he can adhere to marriage's moral code like straights do. He has urged fellow homosexual activists "to fight for same-sex marriage and its benefits and then, once granted, redefine the institution of marriage completely, to demand the right to marry, not as a way of adhering to society's moral codes but rather to debunk a myth and radically alter an archaic institution."
Finally, from a secular or naturalist point of view, taking into account the dramatic rise and fall of homosexual marriages - one must ask as to whether it truly is productive for society. Evolution teaches survival of the fittest. Yet the vast majority of deadly sexually transmitted diseases run rampant in the homosexual community. Leading to death. How does this equate to survival of the fittest? Simply put; it does not.
Any secular argument against same-sex marriage also applies to single parents because it always comes down to father figure/female figure needed in child's development. I have ran into some Catholic groups that countered this by saying single parents shouldn't be allowed to have children, which just made their position more legitimate but also more preposterous.
Technically speaking, we cannot have any secular comments regarding the sanctity of marriage, because marriage is not intended to be a secular institution. Marriage is a religious institution, so discussing marriage in any political forum defies the separation of church and state. Politically speaking, the government has no authority or responsibility to honor marriage. Civil unions is the only real "marriage" that should be discussed at the secular or political levels.