"Bentnail had intimated to him that the unnatural crimes of Sodom were visited upon the boy by Boerhaave & his 'garter-snakes'. "

There are a surprising number of academic books and articles which explore the history of sodomy and homosexuality in the navy, which (at an official level at least) was heavily proscribed and potentially punishable my death. Generally heterosexual licentiousness in port was de rigueur, although to what extent homoerotic activity was tolerated onboard is a matter of some debate. Eighteenth-century British military naval records indicate that allegations made against high-ranking officers by cabin boys were usually dismissed. In Cloud Atlas the two cases of the rape of a minor occur within the context of a work that explores the theme of predation in its many forms.

 

The Destruction of Sodom and Gomorrah, John Martin, 1852.
Public DomainThe Destruction of Sodom and Gomorrah, John Martin, 1852.

His choice of insult 'garter snakes' is especially apt. The garter snake feeds on weaker animals, and is in some cases capable of giving out female pheromones, thus luring other males into attempting to mate with them.