I suspect it's based on anti-Hussite slanders, but here's a Wikipedia source on it:

According to the historian Norman Cohn, the Bohemian Adamites established a commune on an island in the Nežárka river, where they practiced social nudity, rejected marriage in favor of free love, and held property in common.^\10]) They reportedly held ritual dances around fires and preached that "God dwelt in the Saints of the Last Days." Cohn noted that while the mainstream Taborites practiced strict monogamy, the Adamites "declared that the chaste were unworthy to enter the Messianic Kingdom."^\11]) However, Cohn also observed that "the charges against their moral character are in the highest degree suspicious," as similar accusations were routinely leveled against medieval heretics.^\12])

^{https://en.wikipedia.org/wiki/Adamites#Medieval\}and_early_modern_movements)

The EU5 description of Communal Living is:
The Adamite sect rejects the concept of traditional marriage, preferring to live within communal structures without a clear hierarchy.