Ok, let's try this.
Outside of religious belief, what are the reasons for denying the right to marry to a portion of our citizens?
If it makes sense not to allow gays to marry, it will make sense outside of a religious context.
First, nobody is denied the right to marry. If anything, gays already enjoy special protections under the law that are denied to straight individuals. Homosexuals have MORE rights, not less.
Second, rather than asking why we shouldn't change anything, why not tell me why we should be forced to change things? Most Americans find homosexual marriage to be objectionable (and this is not necessarily a religious objection). If you wanna make a change, and in the process force most of your contrymen and women to accept something they strongly disagree with, then the burden is on you to give compelling reasons why. You have to explain BOTH the reasons why the change needs to be made, AND exactly which delegated authority grants government power to force those changes upon all of us.
Third, once marriage is expanded to allow two men or two women to marry, we won't be able to stop there. Some men love more than one women, so let's also expand marriage to accomodate folks who want three-way marriages. Some men love little girls, and in the interests of not discriminating against them we'll have to expand marriage to allow old guys to marry 8 year olds. The single women living next door really loves her cats. She's a nice person, and we can't discriminate against her, so we'll have to expand marriage to allow unions with one's pets (it wouldn't be fair to deny Mischieff and Fluffy and Figaro access to their "domestic partner's" health coverage). Of course, many couples love each other dearly, but only for a few weeks or months at a time. Let's stop insisting that marriage be a life-long affair, and thus allow all American couples the right to marry regardless of their level of commitment. If the un-insured kid next door breaks his arm, perhaps you should be allowed to marry him for the afternoon, so that his injury is covered by your own employee benefits (nevermind that your employer never intended to pay for converage that extends to the entire neighborhood).
"Marrying" you cat is absurd. But really, it's no more absurd than "marrying" someone of the same sex, or "marrying" your 5 best friends, or "marrying" God-only-knows who or what else...
Blech. I have half a dozen non-religious reasons why gay "marriage" is a bad thing, but it's late and I'm tired. I'm going to bed. Goodnight, y'all.