The notion of hieros gamos does not presuppose actual performance in ritual, but is also used in purely symbolic or mythological context, notably in alchemy and hence in Jungian psychology.
In Greek mythology the classic instance is the wedding of Zeus and Hera celebrated at the Heraion of Samos, and doubtless its architectural and cultural predecessors. Some scholars would restrict the term to reenactments, but most accept its extension to real or simulated union in the promotion of fertility: such an ancient union of Demeter with Iasion, enacted in a thrice-plowed furrow, a primitive aspect of a sexually - active Demeter reported by Hesiod, is sited in Crete, origin of much early Greek myth.
In reality, to what extent such a sacred marriage was not just a way of viewing nature, but an act expressed or hinted at in ritual is difficult to say. The best-known ritual example surviving in classical Greece is the hieros gamos enacted at the Anthesteria by the wife of the Archon basileus, the "Archon King" in Athens, originally therefore the queen of Athens, with Dionysus, presumably represented by his priest or the basileus himself, in the Boukoleion in the Agora.
The brief fertilizing mystical union engenders Dionysus, and doubled unions, of a god and of a mortal man on one night, result, through telegony, in the semi-divine nature of Greek heroes such as Theseus and Heracles among others.