"Disruption" of the "nuclear family" comes from the Marxist view that a monogamous family unit leads to the holding of property. In earlier non-monogamous societies, only the women knew for sure which children were theirs. That meant that property tended to accrue to communities formed around matriarchs, rather than single families.
As a father, if you didn't know who your child was, you protected them all, and had to cede property to the matriarchs.
Engels wrote that this was why "the patriarchy" should be fought and "dismantled" because it led to property ownership, and sensitivity against redistribution. Therefore, this remains a core value of modern Marxists, to move society back to fatherlessness so that establishment of redistribution mechanisms is more palatable.
Seems like a fairly horrific goal to me. Property rights have led to the single greatest increase in standard of living for all of Western civilization. Seems like we would want more people joining the system than tearing it down.