Was Io assaulted, or did she have a consensual affair with Zeus? Or does it vary in sources, like Callisto's case?
It varied. It was sometimes said that Zeus and Io never had sex, and that their child was born entirely asexually (and in some cases, via pure divine light impregnating Io). In other cases, she was raped; though Prometheus Bound has the most intriguing account imo, following the idea that she and Zeus never had sex, but rather than this being a lie by Zeus-- the two literally never meet until Io is human again at the end of her journey.
[dream visions spoke to Io each night:] “You are a very fortunate young girl, so why remain a virgin all this time, when you could have the finest match of all? For Zeus, smitten by the shaft of passion, now burns for you and wishes to make love. Mychild, do not reject the bed of Zeus, but go to Lerna’s fertile meadowlands, to your father’s flocks and stalls of oxen, so Zeus’s eyes can ease his fierce desire.”
Visions like that upset me every night, till I got brave enough to tell my father about what I was seeing in my dreams. Hesent many messengers to Delphi and Dodona [...] [The river god] Inachus [her father] at last received a clear response, a simple order which he must obey— to drive me from my home and native land, to turn me out and force me into exile, roaming the remotest regions of the earth— and if he was unwilling, Zeus would send a flaming thunderbolt which would destroy his entire race, not leaving one alive.
Inachus' "entire race" would include literally everyone in Argos, since he was their ancestral forefather and probably a part of their local dodecatheon or something. The story gets even more interesting by not mentioning an initial encounter between Zeus and Io at all. She was immediately transformed into a cow, presumably by Hera to keep Zeus from getting her:
He [Inachus] did not want to do it but was forced by the controlling power of Zeus. Immediately my mind and shape were changed. My head acquired these horns, as you can see, and a vicious fly began tormenting me with such ferocious stings I ran away, madly bounding off to the flowing stream of sweet Cherchneia and then to Lerna’s springs. But the herdsman Argus, a child of Earth, whose rage is violent, came after me, with all those close-packed eyes of his, searching for my tracks. But an unexpected fate which no one could foresee robbed him of his life. And now, tormented by this stinging gadfly*, a scourge from god, I am being driven from place to place.
Earlier she mentions that Argus had been metamorphosed after death into the gadfly, this story predates Hera's association with the peacock, so that makes sense
That gadfly stings me once again, the ghost of earth-born Argus! Get him away from me, O Gaia, that herdsman with a thousand eyes— the very sight of him fills me with terror! Those crafty eyes of his keep following me. Though dead, he is not hidden underground, but moves out from the shades beneath the earth and hunts me down and, in my wretched state, drives me to wander without nourishment along the sandy shore beside the sea.
(italics to indicate that this preceeds the rest of Io telling her story)
After the previous mention of her being sent around from place to place, Io's sisters, the Inachidae naiads, weep for her while they and she all meet with Prometheus. Skipping ahead a bit, Prometheus foresees the troubles Io will face, including an episode with the Amazons:
...Then cross those [Caucasian] peaks, which stretch up to the stars, and take the path going south, until you reach the Amazons, a tribe which hates all men. In days to come, they will found settlements in Themiscyra, beside the Thermodon, where the jagged rocks of Salmydessus face the sea and offer sailors and their ships a savage welcome. They will be pleased to guide you on your way...
Ultimately, the author places the blame of all of Io's troubles on Zeus, for desiring sex with Io, he tore her from her home and set her about to such terrible lands full of danger (other than the Amazons, who seem to be pretty chill with her compared to the others lol). This is fitting, considering the author regards Zeus as very unjust, in a way that is startlingly modern for the late 5th century BC, and was (so I have heard, on the grapevine) inspired in many ways by mid 5th century comedy plays like those of Aristophanes. As proof of that first thing I said, here is one of the first things Io says:
What did I ever do, O son of Cronos, how did I go wrong, that you should yoke me to such agonies and by oppressing me like this, setting a fearful stinging fly to chase a helpless girl, drive me to this madness? Burn me with fire, or bury me in earth, or feed me to the monsters of the sea. Do not refuse these prayers of mine, my lord!
IO: What’s that? Will Zeus’s power be overthrown? PROMETHEUS: It seems to me that if that came about you would be pleased. IO: Why not? Because of him I suffer horribly.
Then back to Io's story as a whole:
[Later, Prometheus:] ...Now hear about another fearful sight. Keep watching out for gryphons, hounds of Zeus, who have sharp beaks and never bark out loud, and for that one-eyed Arimaspian horde on horseback, who live beside the flow of Pluton’s gold-rich stream...
...On the very edges of the mainland, where at its mouth the Nile deposits soil, there is a city—Canopus. There Zeus will finally restore you to your senses by merely stroking and caressing you with his non-threatening hand. After that, you will give birth to dark-skinned Epaphus ["Touch"], named from the way he was conceived by Zeus, and he will harvest all the fruit that grows in regions watered by the flowing Nile.
Hera is occassionally cited as being angry at Io, but primarily the blame is placed on Zeus for lusting after Io. Hera's apparent actions/feelings towards Io are quite confusing in the story, since we're missing the piece of Io and Zeus ever having any sort of correspondence.
[Prometheus:] ...She [Io] fires Zeus’s heart with sexual lust, and now, worn down by Hera’s hate, is forced to roam around on paths that never end.
[Io:] . . . Aaaiii! . . . I have come rushing here, wracked with driving pangs of hunger, overwhelmed by Hera’s plans for her revenge. Of those who are in misery . . . Aaaiiii! . . . which ones go through the sufferings I face? Give me some clear sign how much more agony I have to bear!
Revenge of what? Nothing has even happened. She was immediately transformed after being kicked out of her house. Argus wasn't even killed as Io's prison warden or whatever, he seemed to be hunting Io down and terrifying her, until Hera turned him into a gadfly after he was killed somehow (presumably by Hermes).
[Prometheus:] ...Now listen to what she has yet to face, the ordeals this girl must still experience at Hera’s hands.
[Chorus of Io's sisters:] ...O you Fates, may you never, never see me going as Zeus’s partner to his bed, and may I never be the wedded bride of anyone from heaven. I shake with fear to look on this unmarried girl, young Io, so devastated by the cruel journey, her punishment from goddess Hera. For me, when a married couple stands on equal footing, there is no cause to fear and I amnot afraid. So may the love of mightier gods never cast on me that glance which no one can withstand. That is a battle where there is no fight, where what cannot be done is possible. I do not know what would become of me, for I can see no way I could escape the skilled resourcefulness of Zeus.
Holy yap, rapid fire time:
Cercops's Aegimius (6th century BC) poem (elsewise known as "the poem where Thetis does a lot of infanticide" and "one of the only Greek sources of Theseus choosing to abandon Ariadne") didn't seem to explicitly mention Io's thoughts:
and Hera set upon her as a guard Argus, strong and great, who watched with his four eyes, on this side and on that, and the goddess gave him tireless strength, nor did sleepfall upon his eyes, and he kept watch steadfast forever...
then a summary of what Cercops wrote:
Hera asked Zeus for the cow for herself and set as its guard all-seeing Argus [...] Cercops (scil. says that he was the son) of Argus and of Asopus’ daughter Ismene.
then Ps.Hesiod's Catalogue of Women (6th century BC) summary, which was apparently repeated or coincided by Acusilaus (6th century BC), which was either a rape or at least a kidnapping:
Zeus raped her while she was the priestess of Hera. When he was caught by Hera he laid hold of the girl and transformed her into a white heifer, and swore that he had not had intercourse with this female: for this reason Hesiod says that oaths sworn for the sake of Eros do not draw down the wrath of the gods. But Hera asked for the heifer from Zeus and set up the all-seeing Argus as guard over her.
Bacchylides (5th century BC), where it's unclear how Io felt about him, but at the very least, they did have sex beforehand:
There was a time when by the counsels of wide-powered Zeus the golden cow had left Argos, land of horses, and was in flight—the rose-fingered daughter of Inachus; when Argus, looking from all sides with tireless eyes, was ordered by the great queen, gold-robed Hera, to guard unresting and unsleeping the lovely-horned heifer, and Maia’s son could elude him neither in the bright daytime nor in the holy night. [...] for Io <driven by the gadfly’s sting> reached the flowery Nile, carrying <in her womb> Epaphus, child <of Zeus>; there <she gave birth to> him, ruler of the linen-robed <Egyptians>, abounding in exceptional <honour>, and <brought to light> the mightest <line> among mortals.
Finally, true Aeschylus in his Suppliants, which preceeded Prometheus Bound. I'm lazy and not gonna indent it, it's rather long:
chorus They say that once upon a time a certain Io was keyholder of the temple of Hera in this land of Argos. pelasgus She certainly was; that is the general and dominant tradition. chorus Is there perhaps also a story about Zeus making love to a mortal? pelasgus Yes, and their embraces did not remain concealed from Hera. […] pelasgus How, then, did this quarrel between the royal pair end? chorus The Argive goddess turned the woman into a cow.
pelasgus So did Zeus couple again with this cow with the beautiful horns?
chorus They say he did, mounting her in the shape and likeness of a bull.
pelasgus And what did the powerful consort of Zeus do in response to that?
chorus She set over the cow a watchman who could see everything. 60
pelasgus Who is this all-seeing lone cowherd you speak of?
chorus Argus, child of Earth, whom Hermes slew.
pelasgus What else, then, did she contrive against the unfortunate cow?
chorus cattle-driver.
pelasgus the gadfly that forces to keep moving?
[...]
chorus Yes, and Zeus the Toucher begot a child for her by the touch of his hand.
pelasgus So what does the cow’s Zeus-begotten calf claim to be called?
chorus Epaphus, rightly named after her deliverance.
wait one more, Apollodorus bc it's funny:
...she came to Egypt, where she recovered her original form and gave birth to a son Epaphus beside the river Nile. Him Hera besought the Curetes to make away with, and make away with him they did. When Zeus learned of it, he slew the Curetes; but Io set out in search of the child. She roamed all over Syria, because there it was revealed to her that the wife of the king of Byblus was nursing her son; and having found Epaphus she came to Egypt...
Hera made about machinations to kidnap Io's son but, unlike Ps.Hyginus who said that she killed him, Hera straight up just kidnapped Epaphus and gave him to some Syrian royal family, which is really fun. Reminds me of how she wanted to "kidnap" Dionysus as a baby in the Bacchae, but didn't actually try to harm the clone of the baby once she got him.
anyway the Zeus/Hera/Io dynamic is really complicated and interesting. Io is basically always a victim, but whether that's from physical torture, sexual unwillingness, or both, varies. Also which god is ultimately responsible varies. Hera turning Io into a cow to keep her away from Zeus is my favorite version of the story, bc it just feels like she's trying to keep Io from staying in one place and getting caught by Zeus.