"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?