Your response is fine, and I agree with LazerA's answer.
The only thing I would add, is that it depends on the person asking the question. If the questioner has a genuine curiosity as to why different Jews do different things, I think it's appropriate to go into a little more detail.
You might say, "For me, there's more to consider than the meat and cheese together. I don't know if the cheese itself is kosher. Even if it were, the people making the pizza put their hands into all of the meat, cheese, and vegetables, and there may be some accidental crossover. Furthermore, even if the cheese was kosher, and there was absolutely no cross-contamination, the oven itself can't be used for kosher food, because it has been used to cook non-kosher meat products, and the heat of the oven transfers the flavor and aroma of the food throughout the entire oven."
At that point, if he says, "That's very interesting, thanks for explaining it to me. So why isn't Josh concerned about all of that?"
Then you are back to, "I don't know, you would have to ask him".