Alas, a blog notes that anti-Same Sex Marriage forces have changed their view of the purpose of marriage from a life-long, loving bond between two people to focusing on marriage as a baby-factory.
In large part, the shift in the argument has come about because of recent Supreme Court decisions. In Lawrence v. Texas, the Court struck down a statute outlawing sodomy. The court said moral disapproval alone is not enough to justify a law. Earlier in Romer the Court stuck down Colorado's Amendment 2, which would have repealed all laws that protected citizens from discrimination based on sexual preference. The Court said the law was so sweeping that it was clearly "born of animosity" toward homosexuals.
These cases rule out the must of the real reasons behind keeping marriage exclusive: homosexuality is immoral; treating homosexuals equally will remove a government-enforced stigma against same sex couples. The desire to keep marriage exclusive has its roots in religious faith and taboo, but the Court will not uphold a law based solely on religion or irrational hatred. Thus, secular reasons need to be promoted to the forefront of the debate.
And looking for reasons to exclude same sex couples from marriage, not based on, "It's always been this way," "God said so!" or disapproval of homosexuality itself, are difficult to find.