Sure, couples of the same religion have many resources to maintain the benefits of the family without resorting to legal means. But the religious should not be the only ones able to benefit from marriage.
By and large, homosexual activists do not want to be married, they want to destroy marriage because they think it "patriarchal" and "heterosexist." Abolishing marriage because of the legal threat of deviants getting in on the act is a surrender--indeed, it is defeat. The battle's just getting started, why put up the white flag now?
It is precisely the opposite. Governments love to legislate, regulate and control. Government's intrusion into marriage allows it to project its power into the most sacred and private aspects of a person's life. Indeed, it makes governemnt itself a party to the marriage. Government dictates the terms of the marriage. It writes the contract for you, and would give you the choice of taking its terms or not marrying at all.
Witness the myriad of marriage laws. And the power of courts over married people and those who wish to leave marriage. Government will tell to wait 3 days or seek its council before it *allows* you to be married. It taxes you money (marriage license fee), for the *privilidge* of its almighty blessing. Indeed, it claims that it and it alone possesses the power to make two people married. Such arrogance!
Further, non-religous and mixed-religious homes will lack the stability of law that state-sanctioned marriage provides--and that lack of stability will show in the moral and mental health of such unions' children.
The state's involvement in marriage encourages women to leave their husbands for often trivial reasons, knowing that the state will order the husband to continue support. It absolutely will not enforce visitation orders. It encourages baseless restraining orders (often they are standard issue with the divorce) against husbands. The state's marriage apparatus has been instrumental for the femenist movement to drive a wedge between husbands and wives, and between fathers and children.
But the religious should not be the only ones able to benefit from marriage.
The benefits of marriage do not come from the state. The state may grant priviledges to married people, but these are exceptions to rules the state is responsible for in the first place. And state's legal deference to married couples in economic matters is a form of socialism.