Why wasn't Poseidon turned into stone?

Question

"She [Medousa], it's said, was violated in Minerva's [Athena's] shrine by the Lord of the Sea (Rector Pelagi) [Poseidon]. Jove's [Zeus'] daughter turned away and covered with her shield her virgin's eyes. And then for fitting punishment transformed the Gorgo's lovely hair to loathsome snakes."

So Poseidon most probably looked at Medusa during or after the transition.

Perseus delaying resolutely, and combining that with calm words. Inferior in strength (who could equal Atlas in strength?), he said, ‘Well now, since you show me so little kindness, accept a gift’ and turning away himself, he held out Medusa’s foul head, on his left hand side. Atlas became a mountain, as huge as he himself had been. Now his hair and beard were changed into trees, his shoulders and hands into ridges. What had been his head before was the crest on the mountain summit. His bones became stones. Then he grew to an immense height in every part (so you gods determined) and the whole sky, with its many stars, rested on him.

Gods could also be turned into stone, so why wasn't Poseidon a stone forever?

Answer 1

The actual tale as told by Ovid is a little different from the version you quote. This is the only ancient source that I know of for this version of the Medusa story, and he tells it like this:

And yet, before they were aware, the tale
was ended; he was silent. Then rejoined
a noble with enquiry why alone
of those three sisters, snakes were interspersed
in dread Medusa's locks. And he replied:—
“Because, O Stranger, it is your desire
to learn what worthy is for me to tell,
hear ye the cause: Beyond all others she
was famed for beauty, and the envious hope
of many suitors. Words would fail to tell
the glory of her hair, most wonderful
of all her charms—A friend declared to me
he saw its lovely splendour. Fame declares
the Sovereign of the Sea attained her love
in chaste Minerva's temple. While enraged
she turned her head away and held her shield
before her eyes. To punish that great crime
Minerva changed the Gorgon's splendid hair
to serpents horrible. And now to strike
her foes with fear, she wears upon her breast
those awful vipers—creatures of her rage.

In Hesiod's version Poseidon and Medusa also had a thing: "With her lay the Dark-haired One in a soft meadow amid spring flowers" but he doesn't mention snaky hair or any punishment, Medusa is just the mortal sister of the two immortal Gorgons.

Now you might well ask why Poseidon wasn't turned to stone, to which I can only reply that he wasn't. There seems to be a tension in the myth between beautiful Medusa and ugly Gorgon, and Ovid's story was probably one rationalization of this problem. (This was bothering people back then, too.)

Poseidon also raped Demeter the Furious while she was looking her for her daughter, and he and an actual Fury also had a liaison, so I'm guessing that maybe he simply likes scary women.