Greeks, in addition to their emphasis on the health of the mind and their contributions to philosophy, were also the originators of the Olympic tradition and valued health and beauty of the body. Idealism being one of the key trademarks of Classical sculpture, and the Olympic games being performed nude, we are led to Exhibit A:
Discobolos, by Myron.
Greek preference for balance and idealism are seen here in the capturing of a pivotal and athletic moment made perfect by the way Myron has formed a perfect coil of the thrower's body. We know exactly what is going to happen next, but we can also see the male form displayed to its fullest beauty.
Now we move to open eroticism. With the Venus of Knidos (Exhibit B):
and the famous Venus de Milo (Exhibit C):
We see the first instances of the nude female. Seriously, we covered history, and these are the first nude females since the Paleolithic. Venus of Knidos is about to step into her bath, her hand covering her pubis in what is ostensibly modesty, but in a gesture that ultimately leads the viewer's eyes in that direction. Also is the fact that we can see the clothes she has just removed in her other hand, which makes their absence from her body all the more conspicuous. The Venus de Milo has a more overt eroticism, since we see that her clothes are and have been in the process of falling for about 2200 years.
But, MALE eroticism. Exhibit D:
A Sleeping Satyr. His arm reaches up in a lazily suggestive gesture, and we know, somehow, that he's been drunk and is now sleeping it off. Unlike Venus of Knidos, the sayr is unaware of the presence of a viewer, and his unconsciousness make the image all that more erotic. There is also the unmistakeable spread of the legs drawing the attention of the viewer quite definitively toward his genitals.
In Greece and Rome, there was no shame in homosexuality. It was celebrated and revered, and Greek history is the story of relationships between older men and younger boys. Exhibit E:
Also, remember Achilles? Big bad Brad Pitt from "Troy," who was such a cry baby that he refused to defend his country just because Agamemnon stole his concubine? His relationship with Patroclus (the guy who steals his armour and then Hector accidentally thinks he's killed Achilles) is one of the most famous relationships in all of Greek canon. Exhibit F:
There they are.
In addition, the subject of sexuality was one Greco-Roman culture was open to exploring, even in the sad and perverse. Here is a statue of a hermaphrodite from the Borghese Gallery in Rome (Exhibit G):
You're right, however, that Renaissance forward art seems to be more preoccupied with the female form that the male form. Note the thematic reoccurence of this Female position (Exhibits H, I, J, and K, Giorgione, Titian, Goya, and Manet, respectively):
Notice also the recurring position of the hand, as in the Venus of Knidos above: erotic modesty.
My theory on this is that homosexuality simply wasn't a concern in that time period. Men didn't ignore the male form because they thouht people might think they were gay. To think that during the Renaissance would be absurd. It simply wasn't on people's minds. I'm not at all saying that homosexuality died out completely during the time period. Simply that the animal tendency of men is to love women, because we preserve the species. The Greek sense was one of balance. Everyone loved everyone else in a balanced way: Men and men, men and women, women and women (we're talking Sappho here). In the Renaissance, conservatism and Christian doctrine were key. Also, women in Greece held a far lower place in society than Renaissance women (which is really saying something, but is true). Renaissance women were usually free to go about their daily business, but Greek women, especially in Athens, were forcibly confined to the house. There is also the more recent preoccupation with female virginity and pure bloodlines. The Renaissance follows direcly on the heels of the Hundred Years War, in which bloodlines are key.
Greece was ruled by democracy (sort of), and Roman emperors tended to adopt unrelated successors or overthrow each other. Female purity had very little to do with it. However, by 1450, the Valois, the Lancasters (and Tudors), indeed the entire feudal system had been firmly in place for centuries, and wars were fought over who had the blood right to inherit the throne. Hence, more sexual restrictions upon women. Couldn't have the queen sleeping around with everyone, then who would be the father of the prince? Someone could have claim over the throne. Hence, it was also more forbidden for women to be naked, hence the attraction for constantly painting them as such.
Males WERE portrayed in the Renaissance, to a certin extent (Exhibit L):
Michelangelo's famous "David."
As far as male eroticism, I refer you to Exhibit M, a Botticelli:
Look back at the sleeping satyr. The same erotic laziness, asleep, unconscious that Venus (like the viewer) is watching him. He is covered only by a piece of cloth, while Venus is fully clothed. But they've just had sex, no? Also notice that position of his hand. Look familiar?
That's all I have, because it grows exceeding late.