Yes, that's exactly the problem--rendering the Tomb of King David ritually impure. But of course Catholicism doesn't recognize Halakhah. I can understand why they wanted to have a mass there . . . it's supposed to be the exact room where the very first mass was said. And at the time, I don't think there was a tomb underneath it (I'm not sure, but I think David's body was brought there later from its original burial place).
This is simply a case of one religion being right and the other being wrong--or one true and the other false. American Fundamentalist chrstians don't like to think of it this way, but that's the way it actually is.
Hold your horses! I am an American Evangelical Christian, who happens to acknowledge that Jesus was a devout Jew. I also believe in all the promises made in the Tanakh to Abraham and his descendants for eternity, so I reckon that makes me an Evangelical Zionist. But I disagree with your statement that one religion is true and the other false. There are many things I think are wrong in Catholicism, but overall there should not be hard feelings between Christians and Jews. Jews are the original olive tree and we are wild branches grafted into it. My point was that since that building is the tomb of a Jewish king, located in the Jewish capital, and being used by Jews, I thought it was wrong to do anything there that would go against Halakhah. It has nothing to do with being right or wrong, it is basic courtesy and acknowledgment that you were there first and have all rights to the location. I would NEVER agree to celebrate Mass or any other Christian service there, out of respect to our older brothers in the faith.