Zeus himself had made the offer and Ganymede accepted.
Before meeting Ganymede, the god found himself in a ‘marriage of necessity’. Nothing stopped him. Anything and everything was the target of his lust and nothing could satisfy him until Ganymede.
Now there would be no more bastard half gods and Zeus stayed at home, proving that a good slave can change the world. No wonder they made him immortal. If he could do THAT to Zeus then he held a power beyond even the gods because none of them could solve the problem. Yet along came the right young man that kept Zeus happy.
Perhaps Ganymede offered something so simple, it had escaped them. Perhaps buried in his pure young heart was the comfort & support that out-shown all the flashy others. Perhaps Ganymede was simple and good, like this simple baked corn dish offered here.