The most "definitive" studies that report on the question were the Kinsey surveys, but they're mostly 60 years old, and subject to some questions about general validity even when they were more current. There were at least three major "Kinsey Reports," but only one actually surveyed people out in public. The two earliest used mostly results from prison populations.

Masters and Johnson reported some figures, but since their main interest was in treatment of persons suffering "malperformance" difficulties, there is considerable question of how applicable their results are to any general population.

Other "highly praised" surveys that have been reported, and even cited in some medical research reports, have mostly been of "self-selected" populations, such as readers of a particular magazine or some other subgroup population. Surveys conducted specifically for scientific data gathering by researchers using accepted/acceptable methodology to get information on the population at large have been mostly too small to gather really good samples.

And on surveys, people lie.

So the real answer is that from a scientific/medical standpoint, nobody really knows. But speculation is a common parlor/party game.

There is a distinction to be made between which have more affairs, and which would like to have more affairs; and failure to separate these subtly(?) different categories of replies has polluted a number of otherwise marginally useful surveys.

John