But the males without the gene are indistinguishable from those with it. They reproduce at exactly the same rate - there is no selection pressure for normal mal

But the males without the gene are indistinguishable from those with it. They reproduce at exactly the same rate - there is no selection pressure for normal males. You can try it in computer simulations if you like. It would be trivial to set up. Each generation the percent of normal males decreases. Eventually they are all gone.