Middle age sculpture does not indulge into nudity as subsequent sculpture.
Michelangelo is just one of a long string of artists who celebrated Man as a divine creature as it is, its form being his beauty and his glory, be it in the bloom of youth or in the decay of old age. That's all Umanesimo, putting God-created Man at the centre of creation and at the centre of attention and interest, Man as the highest creation of God, and for that reason sacred in itself.
Omnia munda mundis, all is pure for the pure of mind (and consequently all is dirty for the dirty of mind).
Those who only think about sex end up being "puritans".
In "Sacred and Profane Love" by Titian who impersonates the "sacred" love and who impersonates "profane"?
The answer is inside yourself.