But cheating on spouses in general undermines the trust that spouses should have in each other, and the cumulative impact of even 1% of spouses cheating on the institution of marriage as a whole could be quite negative.

In the comments on Scott's blog, I've recently seen the claim that this is the opposite of how traditional marriage actually worked; there used to be a lot more adultery in old times, and it acted as a pressure valve for people who would've divorced nowdays, but naturally it was all swept under the rug.