My father came from good Protestant Afrikaner stock, so it came as a surprise to his family when he married an English-speaking Catholic woman. His children held true to form; even though we grew up in an Afrikaans town, both his sons and his daughter married English-speaking people. In my case, I married an English woman.
There’s evolutionary advantage for children to marry in the same way their parents did – something must’ve worked, since the parents evidently reproduced successfully – but in this case I suspect there’s a more specific trait.
A recessive gene for marrying out could be strengthened by Like Loves Like. When I look at my in-laws, there’s quite a lot of the same behavior at one or even two removes. In my brother’s wife’s family, one sister married an Afrikaner, one married an American, and one a German. My wife’s brother married a Japanese woman; and her brother married a Chinese. If there is a genetic basis for xenophilia, there’s a cross-cultural minority that’s doing its best to keep it strong.