In most interpretations I've heard she became a Gorgon after being a priestess of Athena and breaking her vow of chastity by being raped by Poseidon.
I know their view of rape was sort of archaic (essentially that it destroys a woman) but they also have stories about characters like Philomela which makes it seem like they understand what a horrible crime it is, and that they at least pity victims of it.
What was the Greek opinion of Athena turning one of her priestesses into a monster for getting raped and then helping Perseus murder her? What was the moral lesson?
Was it maybe a mistranslation of a consensual affair? Was it just Athena being sorta mean like she was with Arachne? Or was it "the gods do what the gods want to do"?
In most interpretations I've heard
No. In one version. The only place where this version of the story exists is Ovid, who is not a Greek poet. The gorgon first appears in Homer, where it's just some kind of monster, as she is in Hesiod, for whom all three are the daughters of Phorcys and Ceto. The gorgon, and the killing of the gorgon, is a frequent motif of vase painting and friezes in the Classical and Archaic periods, and she is universally depicted as simply a monster. There is no trace of this story before Ovid, and the story is not picked up by any subsequent authors.
Most of the stories that you're thinking of are either Ovidian or best known through Ovid. Ovid's obsession with rape, which appears with incredible frequency in the stories in the Metamorphoses, is well known, and the subject of great scholarship. However, the question rests on false premises: Ovid created a version of Medusa in which she was the victim of rape, and therefore the Greeks of the Classical Period thought nothing of it.