There are no facts, only interpretations.
Nietzsche

I disagree with Nietzsche, because I think there are facts. But if I agreed with Nietzsche, that would not make me a “Cultural Marxist.”
And I hope that even Nietzsche would agree that a lot of people have a hard time distinguishing facts from opinions.
Facts (such as whether it is raining outside right now) are by definition truth. (I don’t see how it is possible to have intelligent discussions about what is true unless we agree that truth — at minimum — consists of knowable facts.) Opinions sometimes involve questions of fact, but often they involve larger issues over which people disagree. An opinion does not become “the truth” simply because a large number of people share it.
The discussion in the comments below — over objective and subjective truths — brings this to mind, as facts are almost always objective truths, while opinions tend to be subjective.
I think the confusion of fact with opinion is what causes much of the trouble in cultural debates. People who believe very strongly in certain opinions tend to regard these opinions as facts, as truths, and are unable to admit that they are actually opinions. There’s a certain gray area we call “belief” but I’m of the opinion (and therefore believe) that beliefs are opinions. Widely shared and strongly held opinions, but opinions nonetheless. And no matter how strongly held a belief, the fact that it is strongly held does not make it true.
Opinions and beliefs can of course be true. Most of what we call disagreements over truth consist of disagreements not over facts, but over whether certain beliefs are true. This is compounded by the fact that some beliefs are based on perceptions (feelings, if you will), which seem very true to an individual, but which are not provable to others. For example, if someone believes that he has felt the presence of God, that is true for him, but it cannot be called an objective truth — even though it might be true.
If enough people agree that certain opinions and beliefs are true, and they become shared cultural beliefs, while that agreement does not tranform subjective truths into objective truths, it forms the basis of what can be called objective morality. This can be further divided into religious truths such as “keep holy the Sabbath” (said to emanate from God) and ethical truths such as “do no murder” (not dependent on God).
But not all morality is dependent on God — either for belief or for enforcement. Much of what we call “morality” consists of these systems of ethical truths agreed upon over the millenia, and even most atheists would agree that it is wrong to lie, steal, murder, rape, etc. Such social agreements have long formed the basis of Western culture (both from Judeo-Christian and Greco-Roman perspectives), and as they are also formalized into law, it matters very little whether an individual agrees with the philosophical underpinnings, which are derived from religion, philosophy, self interest, and common sense. One does not need to be a Christian to agree with Jesus’s central ethical premise of “Love your neighbor as yourself.”
I realize that I have not begun to articulate a comprehensive system of epistemology, and I’m sure people will disagree with some or all of what I have said. But I don’t think that makes me a “Cultural Marxist.”
UPDATE (03/02/06): I agree with Jeff Goldstein on what I think is an important point:

…it matters not whether you believe classical liberalism to be the manifestation of some metaphysical truth or simply the best possible manifestation of human social contracts.

Unfortunately, this often causes needless debate over the role of God:

… they have but simply to adopt as part of their own philosophical position regarding social contracts the primacy of certain individual rights that are beyond the bounds of any government to remove.

As to why I call this an important point, I should probably admit my bias here. While I believe in God, the religious debate can get very acrimonious. (In the past, it has caused me more than a little grief.)