When a cryptocurrency start-up that promised to revolutionize the fruit and vegetable industry shut down last month, it left behind one word on its website: penis. When a virtual currency company called DateCoin recently tried to entice investors for its initial coin offering, it posted a Facebook ad that featured a reclining woman in a swimsuit with text over her body that read, “Touch my I.C.O.” And after the North American Bitcoin Conference in January, which highlighted 84 male speakers and three women, the official conference party was held at a Miami strip club. Virtual currencies and blockchain, the digital...