I'm not sure about that statistic. When I tried looking into it a while back it looked iffy. I can't remember the details but it had something to do with narrow definitions relating to the convolutions imposed by French restrictions on studies based on categories of race, ethnicity, or religion and thus being forced to rely on immigrant status and country of origin as proxies. In any case you actually do get fairly high rates of intermarriage among second and third generation Latinos and Asians in the US (between a third and half depending on what exactly you're looking at). Black intermarriage rates are still quite low, though rising.