Conserving co-adapted gene complexes.
The suggestion is that a little inbreeding is no bad thing, because it preserves useful combinations of genes that are adapted to your environment.
Ethnies that number in the millions (most of them) would contain sufficient genetic diversity so that one could on the one hand preserve parental kinship and EGI, and conserve co-adapted gene complexes, but on the other hand, find a mate sufficiently different so as to avoid excessive inbreeding.
Certainly, choosing a mate within the same continental racial group is sufficient to meet all criteria. The idea that one needs to engage in inter-racial miscegenation to avoid excessive inbreeding is a complete falsehood, perpetuated by those with an anti-European genocidal agenda.