I don't know what the racial origins are, but I have grown skeptical of many racial explanations, because they're often simplistic, speculative, and often lack a comprehension and awareness of the greater social context and events occurring around the middle of the century, like the intentional weaponization of "racism" against ethnic neighborhoods that were not assimilating into the ever-nebulous category of "whiteness". (Mass migration is one classic instrument, as are highways and bad urban planning. In the first case, mass migration from the South was used to break up ethnic neighborhoods in the North, causing their scattering across the suburbs to hasten assimilation. You could have used practically any outside group, but the convenience here was that it exploited the moral high ground of the civil rights movement, allowing opposition to mass migration to be cast as an expression of racism.)

However, ascribing opposition to "fear of the unknown" as a motivator is likewise lazy. It works from the assumption that a) the nuclear family is arbitrary and thus have no presumption in their favor, and b) that there is no tolerance for substitutes for the nuclear family, in the strict sense. It's not like most people are clamoring to live in a polyamorist sex commune, so it's not a good explanation.