Yes. The only ones that I know of that are definitely plastic are the ZZR1400/ZX12R and some trialie type ones.
*most* other bikes have metal tanks, plastic ones are only usually used when you have the proper fuel tank somewhere else, for example, under the seat (as per ZZR1400/ZX12R) or for a weight saving measure, as per most "proper" trials bikes.
Race reps (such as the ZXR750R) have alloy tanks, which is, again, a weight saving thing.

Hope this helps