How was Andromeda cursed?

Andromeda was not cursed; her mother Cassiopeia boasted that Andromeda was more beautiful than the Nereids - of which the wife of Poseidon, Amphitrite, might have been one - in any case, Poseidon sent a sea monster to ravage the lands until Andromeda's father Cepheus consulted the oracle, who announced that no respite would be found until the king sacrificed his daughter, Andromeda, to the monster. Perseus, returning from slaying Medusa, saw her and fell in love and asked her parents to marry her and they agreed and Perseus killed the monster and freed her.