From a legal basis, strictly speaking, there is no argument that can be devised to dissuade the legalization of gay marriage. Equal protection under the law, is just that.
However gay marriage as law will not provide gays with equal rights, instead it will destroy the legal definition of marriage. Once you can define marriage as being between two people of either sex, the next logical answer is "why stop at just two?" And once that question is asked, marriage ceases to exist as a legal entity.
You see marriage is a religious institution, not a political one. The fact that the government is involved at all is a violation of the first amendment. If the government is so inclined it can devise a contract, call it whatever it pleases, and allow such a contract to allow two or more individuals to enjoy special tax status, and other legal benefits. Beyond this measure, such as ceremony, moral requirements, and social behavior, these things are defined by personal belief, and thus are the realm of religion.
What would you think of the government offering a baptism license?
Communion license? Salvation license? It's appalling isn't it? Yet we concede the government defining marriage for us? If you use the government to enforce your religious beliefs you are no better than any other tyrant.
You see the rise of the acceptance of the term "gay marriage" is not a rise in the civil rights of homosexuals, but a concession of religious freedom. Church goers long ago decided they wanted special favor and gave up some of their freedom to be enslaved to the tax code. Now the consequences of that choice is biting them in the ass, and they don't like it. Suck it up, you brought it on yourselves. If you want to solve it, take the right away from the government, instead of bitching about others trying to get in on your legal perks.
There does exist an argument against gay marriage however, and it's not a religious one, it is one of consequences. History shows us that when a culture accepts homosexual behavior as the equal of heterosexual behavior the society collapses. This is because of Natural Law, and Natural Law dictates that the heterosexual marriage of two individuals out of love is the foundation of society.
This is why we cannot ever tolerate the union of homosexuals to be given the same status as heterosexual marriage. This is not to say that we should condemn homosexual unions, or that we should harass these couples in any manner as a society. So long as they neither force me to respect their behavior, nor make me pay for the consequences of their choices, then what business is it of mine?
What I am referring to is forcing gay marriage to have the same status as straight marriage, forcing churches to perform gay ceremonies, forcing businesses that offer incentives to straight couples to do the same to gay couples, forcing the military to offer the same benefits to gay couples as it does to married couples, and worst of all, allowing homosexual couples to adopt children.
When we allow such unions to adopt children, we cross a line as society. Natural law is absolute, it can only be violated for a short time before the consequences catch up with society. Totalitarian states fall because they violate natural law, anarchy falls because it violates natural law.
Natural law states that it takes a male and a female to produce children, and children are designed to need a father and a mother. Some would argue that heterosexuals who can't have children adopt so why should homosexuals be any different? That, quite frankly, is the dumbest question I've heard yet.
Heterosexuals who can't produce children have a physical ailment preventing the natural course of sexual reproduction. Thus adopting has no negative consequences for the child as they still receive a loving father and mother.
Homosexuals can be perfectly healthy and never produce children because two men or two women can't produce a child. But they are loving as well aren't they? Of course, but they can't provide what the child needs. Just as single parents tend to raise dysfunctional adults, so do gay couples.
Marriage is the foundation of society. Changing the basis of that foundation has severe negative consequences. Homosexuals have significantly higher incidences of depression, suicide, drug and alcohol abuse, high incident of sexually transmitted disease, over 60% of new HIV cases each year, and while only comprising 3% of the population account for an astounding 1/3rd of pedophiles.
These are the consequences of homosexual behavior. When society chooses to elevate such behavior to foundational levels, that society marches to it's destruction. Choices have consequences.