In 1953, Kinsey found that 11% of men had had anal sex within marriage, out of a sample of 5.3K men. (This sample is described “younger white adults with some college education,” so obviously we’re only getting a narrow, appropriately 1950s-esque, slice of the population.)
The male population clocked in at 74.8M (67.1M white males) in 1950. The 2010 Census counted the male population as 151M, a 101%+ growth over 60 years. If that 11% still holds, that means over 16.610M of today’s male population have had anal sex.
Of course, this isn’t a completely pure, apples-to-apples comparison (more like apples-to-pears), so I’d like to come back to this if/when I find more information.