I have to agree with the people who have replied that you are in the wrong place to get a secular based answer. The only one I can come up with is this.
If we allow homosexual marriage we will be granting homosexuals the same status and rights as heterosexuals. The problem with that is instability in homosexual relationships. The last numbers I recall is that only 10% of homosexuals are monogamous for more than just a few years. This will result in further destabilizing the family unit in this country. It is in bad enough shape as it is. The divorce rate among Christians is rediculous not to mention non-Christians. This will amplify in subsequent generations by producing more dysfunctional people. More people who don’t know how to love and be loved. More people who do what they want when they want to do it, even if it hurts others. We have proven in the United States that each generation takes the mistakes of the prior generation a step further. This process will continue until either we correct ourselves or the United States goes the way of past great societies which became inherently selfish.
Thanks and God bless,