What woman was cursed with agelessness because of refusing advances of a Greek god?

In the Movie "The Clash of the Titans" Io says that she was "cursed with agelessness" because she resisted the advances of Poseidon. This is 100% incorrect. Io was the lover Of Zeus, and when Zeus's wife found about that he was having an affair she basically went banana's looking for Io. So Zeus turned Io into a heifer, which is a cow before it has calves, to try protect her. But, Then Zeus's wife, Hera, discovered this and demanded Zeus gave Hera the Heifer to Hera as a present. What could Zeus do but comply so not to arise suspicion.