A complete bipartite graph is unique

You know that each node $x in X$ is connected to each $z in Z$. If you were to assume that there are two bipartite graphs that do not have the same edge set, you can create a contradiction, since both graphs are complete and thus they must have the same edge set.

You could compare both and if an edge is in both edge sets, remove it. You will end up with two empty sets (if an edge is not in one of the sets, the graphs is not complete) and thus have proven that they are equal since it contradicts the assumption that there must be at least one edge in one of the two graphs that is not in the other.