So right now I don’t have any budgies at moment. But if I ever get two I don’t want them to mate if they are a different gender. I wouldn’t put a nesting box but my real question is will they mate without a nesting box?
Then don’t provide them breeding materials, or association on the nest box. That means even for dark holes, they may find your open closet as a nest box. As long you don’t have nest box Budgies can’t lay eggs and breed.