We're probably talking about that blogger who did a regression test on the MIDUS survey results?

http://inductivist.blogspot.com/2011/06/predictors-of-getting-cheated-on.html

Note that "very likely to be cheated on" is not an accurate summary, but indeed Agreeableness turned out to correlate with reporting that your wife had been unfaithful. (although if you're a woman the odds tilt the other way: no correlation for Agreeableness, and a correlation with reported husband fidelity for Conscientiousness)