That question comes with a big 'DEPENDS' as an answer. Where I come from, 'acceptable' depends on whether or not it's a poly relationship. Or if they can get away with it. Or whatever religion you're in. A lot of people also just sort of look the other way. Wifey's getting served by her tennis instructor and doesn't care that Hubby is getting extras from the Nanny or his secretary. Marriage gets old. The same set of genitals is boring after a year, let along a decade or ten.
Air Time
That answer your question?