Invalid argument. Military defense is a legitimate function of government. Social engineering is not. Advocacy of government funding for illegitimate functions belongs in the DUmpster, not here.
Social engineering? Who's the one advocating gay marriage here?
We're talking about preserving the ideals of the Judeo-Christian culture that make the freedom you enjoy possible. Already, marriage is in bad shape in this country. The divorce rate is high, children are born out of wedlock, and the nation suffers because of it. Everyone in America knows someone that has suffered because the institution of marriage has been weakened.
Look at countries that relax their laws on marriage. Their divorces increase, as do their illigitimacy rates. It is pushing them to decline and decay. We need to be pushing in the opposite direction, and making marriage more special, and more protected, not cheaper and more frivolous.
I realize that Libertarians don't care, because to them the individual is all, but it's an irresponsible attitude. If we can't defend marriage, then our culture will not remain viable.