Business is booming for the world's biggest sperm bank, Denmark's Cryos, which is struggling to meet demand despite a soaring number of donors and new offices opening around the world. At Cryos headquarters in Denmark's second-biggest city Aarhus, chief executive Ole Schou smiles broadly, sitting at his desk adorned with pictures of cheerful babies as he speaks of the company's unexpected success. Two years ago, he was ready to shut down the business, or at least move it abroad, because of a Danish tax authority proposal that would have required donors to declare income from their donations, thereby putting their...