I vaguely recall this fable, I dont think I read it directly but I read about it. The point of the story was that a man had done something bad, but was too pure or innocent or something like that, so instead of being put to death, he got turned into a woman. I am almost certain it was of greco(-roman) origin.
-It is not in the metamorphoses, there is a similar story there where Artemis turns a guy into a deer, close enough, but it is not it
-It may have been several people, at once or over time and not just a one time occurance but I am not sure
-There is a somewhat esoteric legend of a woman so beautiful that any man who looks at her turns into a woman, which sounds pretty close to the story I am looking for, but I am sure it is not it.
I think I read about it on a pretty accessible site or maybe on youtube so I am surprised I cannot find it just via google. Asking if anyone here has an idea
There are two characters in Ovid that do full bodily gender transitions: Teiresias (or in Ovid's Latin, Tiresias) and Caeneus.
Here's a Greek version of the Teiresias story from just a short while earlier than Ovid, the pseudo-Apollodorean Library, 3.6.7:
Pherekydes says that he was blinded by Athena; for Chariklo was dear to Athena . . . and Teiresias saw the goddess fully naked, and she took hold of his eyes with her hands and rendered him sightless. And when Chariklo asked her to restore his sight, she could not do so, but by cleansing his ears she caused him to understand every note of birds; and she gave him a staff of cornel-wood, with which he walked like those who see.
But Hesiod says that at Mt Kyllene he beheld snakes copulating and wounded them, and he was turned from a man into a woman, but that on observing the same snakes copulating again, he became a man.
The Hesiodic poem doesn't survive, but we can take this as a reasonably accurate paraphrase. In Ovid the story is at Metamorphoses 3.318-338.
The other Ovidian character, Caeneus, makes a different transition, from female to male, after being raped by Poseidon. His story is in ps.-Apollodoros Library epitome 1.22, and Metamorphoses 12.169-535. But I'm guessing that isn't the story you're looking for.
There is however a book by Nicole Loraux called The experiences of Tiresias: the feminine and the Greek man (English translation 1995), which may be of some interest.
For those wondering, I have since found it - it's the story of Siproites, although it is mostly lost now and only preserved in fragments