Sunstein's stat comes from a 2012 political science paper by Shanto Iyengar, Gaurav Sood, and Yphtach Lelkes, which found that "both Republicans and Democrats increasingly dislike, even loathe, their opponents." Though the parties have grown increasingly polarized in voting behavior, these researchers argued that the increasing animus wasn't based on policy differences, but rather "the rhetoric of political campaigns" and "the tendency of the media to recycle the candidates' negative messages."
It's worth noting that a Pew survey asking a similar question earlier this year found less dramatic levels of disapproval — 22 percent of Republican parents, and 19 percent of Democratic parents said they'd be unhappy. However, that question's hypothetical involved "a member of your immediate family" marrying a partisan foe, not specifically the respondent's child. Still, those levels are higher than the 1960 survey found.