I think you still underestimate the cost to society of the loss of such laws at the hands of the secularizing expurgators and moral termites.
Further to your exception to the use of moral standards in arguments about policy, I thought you might like to read Lord Devlin's comments quoted and discussed in this new FR thread:
Yes, the gays have a political agenda. These days, who doesn't? There are only three potential alternatives: power by force, power by political process, or no power. I've heard a few on the gay marriage threads advocate some level of force to "take back the courts", but I expect that they will either not try, or not succeed. That leaves lobbying for political power, or being swept aside to powerlessness.
If those who oppose gay marriage want to avoid the latter, then they had better find an effective method of achieving the former. There may be ways of making the mushy middle see most all gay and lesbian people as promiscuous, disease-ridden, benefit-grubbing, deceiving child molesters, but you'd better hurry, and be quite effective at it, because by 2005 it will probably be way too late.