Figures here from longrunning survey.
Adultery, at least in the US, is becoming less acceptable, not more.
Having always been more on the drink-of-water side of the argument myself I find the belief mildly odd in itself; having always been vaguely of the belief that more casual sex was associated with the progressive agenda I find it odd that the stigma hasn't fallen away in the same manner as, say, condemning homosexuality as always wrong.
Having myself committed adultery on every possible occasion (a fairly small number, I have to say, my beliefs on this matter being a good deal less relevant than those of the other prospective parties) I find this almost inexplicable.
What this says, of course, is that I see it as an issue of religious belief and that other people see it as an issue of human relationships, not to say personal authenticity.