Thetis, goddess of the sea, was beautiful, and Zeus wanted her and so did Poseidon. Then both of them drew back.
It was because of one old prophecy. “The son Thetis bears is stronger than his father.” The gods were afraid of a son stronger than themselves.
So the goddess was married to a mortal, Peleus, king of Phthia. All the gods of Olympus came to that wedding.