Greentext

From the above linked article:

In the case of the Israeli kibbutzim (collective farms), children were reared somewhat communally in peer groups, based on age, not biological relations. A study of the marriage patterns of these children later in life revealed that out of the nearly 3,000 marriages that occurred across the kibbutz system, only 14 were between children from the same peer group. Of those 14, none had been reared together during the first six years of life.

And lots of other examples across different cultures that would be consistent with this being an instinctual reaction of humans, rather than a cultural thing that is taught.