It is generally accepted these days that the grand construct of Archaic and Classical Greek myth, with its pantheon of Olympic and chthonic deities, is an amalgamation of local Greek myth (Athena, Poseidon, Zeus the rain-maker, Artemis, Demeter, Persephone, etc) unified and stitched together by Near Eastern cosmological borrowings. Our earliest Greek written sources on mythology and creation, Homer and Hesiod, have in the last 30 or so years been brought into a sort of alignment with contemporary or slightly older Near Eastern Bronze Age texts, especially the great epics of Syria, Canaan, Anatolia, Mesopotamia, and (on the fringe) Egypt. It is possible to talk about an "Asiatic mythological koine" or Burkert's "Near Eastern-Aegean koine" at the end of the Bronze Age. A koine is an area (physical and otherwise) in which disparate ideas contribute to and forge a common expression.

Within this Near Eastern-Aegean koine we find non-Greek inputs concerning creation mixing with local Greek myths. Neither Gaia nor Ouranos can be easily traced back into a dim Indo-European origin, and clearly have their ultimate home in Near Eastern (ie non Indo-European) world-building. The severing of Heaven's penis is, for example, apparently Hittite in origin, as is the underlying notion of divine succession through violent, often sexualized means (cf the Hittite "song of Ullikumi").

Your question asks specifically about the mingling of Heaven and Earth. I do not think we should necessarily conceptualize this myth as specifically incestuous, or focus on that aspect as the core element of the story. Their activity is pre-familial, more primordial than affectionate, sexualized fertility preceding romantic union. Eros is present, but in Hesiod Theogony plays no obvious driving role, and his presence is anomalous compared to the myth's probable non-Greek origin. The myth seems to go directly back to the creation story of Apsu and Tiamat, Old Babylonian, first discovered on several cuneiform tablets in the 19th century in the so-called Library of Ashurbanipal at Nineveh (Mosul, Iraq). The myth is these days called Enûma Eliš/Enūma Elish, based on the first line of what survives. In the myth, Apsu and Tiamat are not necessarily brother and sister, nor are they lovers—they are primordial, pre-creative. So in Hesiod's telling, the first power is Chaos, the "Gap/Gash," and Gaia, neither of which is a child, but simply is. (in Homer, it is the barren expanse of Ocean first, like the primordial, dark waters of the Hebrew beginning). Hesiod's Gaia bears Ouranos herself, with no sexual contact required. They share no kinship or affection, and their mingling is not specifically penetrative or transgressive and certainly not amorous, but necessary and inevitable given their celestial juxtaposition (though at Theogony 138, Hesiod makes Ouranos "lustful"). Even "later" in the mythic cycle, when we come to technically incestuous pairs like Zeus and Hera, there is no emphasis or consequence to those unions. In fact, you can draw a line between sex/reproduction which comes as a matter of course, without love or even lust, and sex which derives directly from carnal desire: any of Zeus' many sexual transgressions, or Aphrodite's tryst with Ares, for example.

The cosmological origins are vague in Greek thinking, perhaps intentionally so, or the result of a basic disinterest. The beginnings and the endings of the universe are not particularly Indo-European themes, and their inclusion in the Greek myth system seems to be imported for a single purpose: they provide a sketched-out backstory for the gods and systems that matter to the Greeks, the present-day Olympians and their recently-defeated adversaries. The Greek view is relatively narrow, present, and references to the beginning and the end are afterthoughts, occasionally necessary but seldom interesting in and of themselves. There are hints of a rich and tragic framework of myth surrounding the Titans like Tethys and Hyperion, but Greek myth-making did not bother much with them, except for Kronos in his role as father of Zeus. The Titans are background, shadowy, not fleshed out, not given voice. In fact, it will be two millennia before John Keats finally gives a voice and a tale to the tragic Titan Hyperion. This makes a lot of sense in the context of the importation or adaptation of a lot of Greek myth from the Near East.