A caveat here: While the latter two data points -- on interracial marriage and gay marriage -- are estimates based on hard data, Pew's poll is based on self-reported bipartisan marriages. Fourteen percent of Republicans and 15 percent of Democrats offered no response to Pew, and it's quite possible many of them (or even some who assume their spouse is in line with them politically) are unwittingly in bipartisan marriages. (The horror!)
But in a country that has grown increasingly polarized -- and quickly -- in recent years, it's perhaps no surprise that so few people would choose to spend the rest of their lives with someone from the other side of the political spectrum.
Because while Americans have quickly warmed to the idea of people marrying people of the same gender or a different race, they're moving in the opposite direction when it comes to bipartisan marriage.