The standard answer is that all of our naive beliefs about the world would be false. You are not actually sitting on a couch, your simulated body-projection is simulated to be sitting on a simulated couch. The very fact we can't tell whether or not we're in the matrix undermines all our knowledge.
The more insightful answer is that even if we're in the matrix, everything about the physical world is still true, there is just a metaphysical fact we are unaware of--namely, that the universe happens to be a simulation. You're still sitting on a couch, and the couch is still made of atoms, and the atoms are still made of subatomic particles and so forth, but it turns out all the subatomic particles are just data structures in the matrix and we didn't know that before. Nothing is undermined.
You could even argue that metaphysical statements are meaningless, though you run into problems going too far that way as well.
IMHO, the moral relativistic approach as you describe it constitutes an interesting mental exercise, but not a useful one per se.
If there is absolutely no hint or proof that something exists, how can it be useful to debate it (apart from intellectual masturbation) ?
Let's say we live in "the matrix" and that there is _absolutely_ no way to get out of it or anything that could hint its existence. Given that assumption, what is the point of debating whether it exists or not ? Even if there really was a matrix, how could it be useful for us to know this piece of information ? That knowledge wouldn't serve any purpose as the said matrix wouldn't in any way interfere with our universe and even less with our mere human lives. Of course, if there was any way to escape the matrix or if it would even subtly interact with our universe, that would be another question.
In the same vein, I dislike the concept of "agnosticism". If we had to be agnostic about everything that is not possible to prove (or even partially, through subtle hints) then we'd have to be agnostic about an amount of things only limited by our collective imagination. For instance, we'd have to be agnostic about "the matrix", the fact that our universe is an "atom" inside a much larger universe, that we're all imagining this universe through an induced dream, etc.
On a side note, this is one of the reason I find it more interesting to debate with theists that believe they have evidence for God than agnostics or theists that base their belief on faith.
PS: I'm not trying to discard philosophy and I actually admire this discipline. All I'm saying is that it should not attempt to answer the wrong questions.