First, allowing gay citizens to marry would force the law to recognize other kinds of marriage, like polygamy, marriage between siblings, etc. Second, gay marriage is wrong because the Bible condemns homosexuality as immoral. The first argument cannot justify continued discrimination against homosexual couples. The latter is a personal matter to which people are entitled, but has no place in a government meant to serve all people.
Marriage is a civil right that most Americans can enjoy. To deny this to couples because of sexual orientation, it must be shown that there is a demonstrable harm to society in failing to do so. No research shows that children raised in gay households suffer harm from it. What matters is that a child has loving, supportive parents, regardless of sexuality.
The rest is a matter of how consenting adults choose to live their lives.
What opponents of marriage equality must answer is the question of discrimination. Not allowing homosexual couples the same rights as heterosexual couples is discrimination based on sexual orientation. Why shouldn’t a gay person be covered under his/her partner’s insurance? Or allowed to visit them in the hospital? Or accept legal custody over their children? These are the questions that opponents of gay marriage must answer.
Refusing to grant gay relationships the same rights as heterosexuals under the law is indefensible. Gay people want to have the right to devote their lives to one another with the same level of commitment as anyone else, and to have the state recognize that devotion. Prejudice is no excuse.