On this question, I and another poster claim on the basis of internet sources that Ganymede was Zeus' male lover. A commenter challenged that, however, and I haven't yet found a reliable source to say that he was. So what is the earliest source to make that claim?
Ganymede's story is told in the Iliad, in the 8th century BC (as the comment refers to):
...and godlike Ganymedes that was born the fairest of mortal men; wherefore the gods caught him up on high to be cupbearer to Zeus by reason of his beauty, that he might dwell with the immortals.
Homer, Illiad, book 20
Ganymede being a romantic/sexual lover of Zeus isn't stated there, though (at least, not overtly). You don't have to look too much further to find that, though.
From the 6th century BC, from Theognis:
The love of boys has given delight ever since Ganymede
Was loved by Kronos' son himself, king of the immortals,
Who seized and brought him up to Olympos and made him
Divine, possessing as he did the lovely bloom of boyhood.
Theognis, fragment, lines 1345 - 1348
And from the 5th century BC, from Pindar:
...then it was that the god of the splendid trident seized you, his mind overcome with desire, and carried you away on his team of golden horses to the highest home of widely-honored Zeus, to which at a later time Ganymede came also, to perform the same service for Zeus.
Pindar, Olympian 1
And in the 4th century BC, from Plato:
...then the fountain of that stream which Zeus, when he was in love with Ganymede, called “desire” flows copiously upon the lover;
Plato, Phaedrus, pg 255
Ovid mentions Jupiter's love for Ganymede him in Metamorphoses X, saying
The king of the gods once burned with love for Phrygian Ganymede, and to win him Jupiter chose to be something other than he was.
That would be about 8 CE. Taken from http://ovid.lib.virginia.edu/trans/Metamorph10.htm#484521421