Back in the days when some j tribes had a beef with gate of gods (Babylon) tradition of marriage was sacred on another level. Different city-states had different patron gods, main reason to have a war. Lugal (big man) of one city-state marrying a saintess from another city-state is a very pleasing event for gods. Now we can be friends, no need to fight anymore.
When saintess is not marrying a hero it's double blasphemy.