There are 61.2 million married men in the United States (Census: 318.9 million people minus the 23.3% under age 18 = 244.6 million adults. About half of U.S. adults are men, and 50.5 of U.S. adults are married, according to Pew).
Divide 61.2 million by 10 million = approximately 1 in 6 U.S. married men were on Ashley Madison.