She was seduced by Poseidon. They made love in the temple of Aphrodite. And that goddess was so jealous that she punished Medusa.
13 seconds sound clip from the Clash of the Titans (1981) movie soundboard.
You can hear this line at 01:14:24 in the Blu-ray version of the movie.
Quote context
[...]
- The sands of time are running like quicksilver.
- We'll be across these mountains tomorrow, by nightfall, nearing the Isle of the Dead.
- And then, Medusa. I wrote a play about her long ago.
- Medusa. She was priestess to Aphrodite.
- Yes, and a most beautiful woman by all accounts.
- It must be done. We've no other choice.
- Tomorrow you return to the city.
[...]