I support gay marriage, but I'll try playing devil's advocate (ironically by siding with hardcore Christians). The definition of marriage is not just about peanuts - it involves the granting of a set of rights and privileges to a particular group of people (eg. favourable tax treatment). In other words, marriage is a subsidy. Generally speaking, governments should only subsidize activities and institutions that serve the public interest. In the case of marriage, governments favour marriage in order to foster an environment conducive to child-birth and child-rearing. Baby bonuses won't necessarily suffice, because it is better for kids be raised by two parents.
Now, gay couples can have kids, or adopt them, but it is difficult for them to do so. Only about 25% of US gay couples have kids, far below the national average. And in some cases, those kids were born in heterosexual marriages, that predate the formation of a gay couple. As a result, gay marriage would mean subsidizing a large number of people, who are far less likely to further the objective of the subsidy. If unequal cannot be reconciled with notions of fairness and equity (or the constitution), the logical solution would be to get the government out of marriage altogether.