Why did it take 46 years to go from Dave Kopay to Carl Nassib?

Kopay and Nassib. Photo illustration by Slate. (Photos by John Prieto/The Denver Post via Getty Images and Julio Aguilar/Getty Images)

On Monday evening, a few hours after the Las Vegas Raiders defensive lineman Carl Nassib became the first active NFL player to come out as a gay man, Outsports, a website devoted to the intersection of sports and LGBTQ issues…