The fact that it exists across all races is the hint that it is genetic. Unless you're suggesting that all races have independently developed identical social pressures to mate with their own race, and override the any inherent genetic tendencies to mate with all races. Then I'd ask how that pressure came to be identical for all races.

I believe it'd be "...identical pressure for all cultural groups" where cultural groups tend to contain (or become?) predominantly one race. I'd argue it's learned behavior because centuries ago, trusting outsiders was risky and what better way to identify "outsiders" than "they don't look like us."?