Two interesting comments recently: one claiming that banning gay marriage denies equal protection under the law, and another asking why lack of judicial review in a parliamentary democracy is abhorrent.
Sometimes bad arguments are made in favor of good policies, and the equal protection argument is one of those. Let me first point out that no state that I am aware of has a law that says "Heterosexuals may marry, but homosexuals may not". What those laws say is that everyone, straight and gay alike, is equally prohibited from marrying someone of the same sex. And in fact, where gay marriage is banned, it's not that uncommon for homosexuals to marry someone of the opposite sex, to gain various financial, practical, and social benefits, and to do so perfectly legally.
Of course the ban has more negative impact on gays than on straights. But all laws affect some people more than others. Consider the following statement:
Any law that prohibits X denies 14th Amendment equal protection to people who want to do X.
That's absurd on its face. X could be anything from rape to bank robbery. At this point maybe you're getting angry that I'm comparing violent, non-consensual acts with peaceful, consensual ones. You're completely correct that there's no comparison - but the difference has nothing to do with equality or lack thereof. You might also point out that homosexuality is something inherent to a person, like their race. True again: a law that banned simply being homosexual would violate the 14th Amendment. But a law that merely bans some things gays might happen to want to do more than straights does not. That's why the SC, in Lawrence v Texas, overturned a sodomy law based on liberty and due process grounds rather than equal protection.
The real reason gays should be allowed to marry each other is because (a) there's no decent reason to disallow it, and (b) it's none of the government's business to regulate consensual relations among adults. Unfortunately those two principles aren't explicitly written into our Constitution, and thus had to be constructed by judges.
Which brings me to the second part about parliamentary democracy. Most people realize that absolutely unlimited democracy isn't a great idea - the old joke about two wolves and a sheep voting on what's for dinner. The error among some otherwise decent people on the left is thinking that coupling two principles, democracy + equality, solves that problem. I believe those two principles aren't sufficient, for the reason given above. You need democracy + equality + limits.
Stinerman made a reference to using violence to solve political differences, and got called on it. Democracy depends on people peacefully , if not happily, accepting the results of elections, the laws passed by elected legislatures, and the lawful attempts of others like FRC to influence elections and legislation. If you want elections to go down peacefully, people have to have confidence that their lives, and the things that make their lives worthwhile, are not at stake. And that means that certain issues need to be taken outside the reach of democracy. I don't know exactly what needs to be on that list, but I do know the list needs to be getting longer rather than shorter.
Judicial review isn't the only way to keep the limits in place, and it's not always effective, but it's better than nothing. Of course the best protection is having legislators, and an electorate, sincerely committed to maintaining the limits agreed upon when the democracy was formed. Sad to say, we lost that commitment some time ago.