ANDROMEDA was the daughter of a foolish king who boasted that she was more beautiful than the NEREIDS. This put her under a lot of emotional pressure — the original Andromeda Strain. These silly claims also put POSEIDON in a really foul mood and he had her chained to a rock to be ravaged and devoured by a sea monster.
So of course it was zap the scaly beast and rescue the maiden in distress time. He regarded this as the ideal heroic courting ritual and went on to marry her. She is now floating around in space as the Andromeda Galaxy and is much more beautiful than the Nereids.