In Ancient Greece, where the mild climate was conducive to being lightly-clothed or nude whenever convenient, and male athletes competed at religious festivals entirely nude, and celebrated the human body, it was perfectly natural for the Greeks to associate the male nude form with triumph, glory, and even moral excellence.
The Greek goddess Aphrodite was a deity whom the Greeks preferred to see clothed.
Christian attitudes cast doubt on the value of the human body, and the Christian emphasis on chastity and celibacy further discouraged depictions of nakedness, even in the few surviving Early Medieval survivals of secular art.
Completely unclothed figures are rare in medieval art, the notable exceptions being Adam and Eve and the damned in Last Judgement scenes, and the ideal forms of Greco-Roman nudes are completely lost, transformed into symbols of shame and sin, weakness and defenselessness.
Male nudes tended to be slim and slight in figure, probably drawing on apprentices used as models, but were increasingly accurately observed.