As briefly as possible:
I copy and paste this from an entry at Right Wing News:
If you read the essay, you'll see that Olson argues mostly on the basis of constitutional law, and he states his case eloquently. But constitutional, states-rights issues are beyond what I want to address here (and I've handled them ad nausea previously). My basic point all along is that on a national controversy this big, it's best to let the will of the voters prevail, for no court ruling will have the same kind of popular legititmacy as that found in a majority vote of the electorate.
And that is precisely the problem with the conservative position on gay marriage. When it comes to issues of constitutional import, and rights that have been clearly delineated as "fundamental" for more than a century, the vote of the populace doesn't trump constitutional considerations. You cannot have a debate about the legality of gay marriage that is disentangled from the Equal Protection issue or the Due Process Clause. This is the entire point of the Bill of Rights and the 14th Amendment. We have a Bill of Rights and an Equal Protection Clause to protect the rights of the minority from the federal government, from our state government, from the police, from the IRS, and yes, from discriminatory referenda by voters themselves.
Simply because a majority of citizens are against gay marriage doesn't mean that gay marriage should be outlawed. As Olson noted, a vast majority of Americans were against interracial marriage at the time of Loving v. Virginia in 1967. A majority of Americans were also in favor of the "separate but equal" policies that were invalidated by Brown v. Board of Education in 1954. These are not novel arguments, but they are arguments that conservatives seem to have great difficulty addressing.
As has been noted here before, I have yet to hear a single conservative take on Olson from a purely legal perspective -- in light of marriage being deemed a fundamental right since the 1880s, in light of Loving v. Virginia, and in light of the Equal Protection Clause of the 14th Amendment -- and make a legally coherent argument against his position. The bottom line is that marriage is a fundamental right, and it is the government's burden to demonstrate that Proposition 8 is critical to advancing a compelling governmental objective that is at least partially secular. These are indisputable principles of constitutional law, and all conservatives want to do is call Olson a turncoat and shriek about judicial activism.
Much more on this later.