It is natural and good
Nature 462, 14 (5 November 2009) | doi:10.1038/462014b; Published online 4 November 2009
Animal behaviour: Fruit-bat fellatio
PLoS One 4, e7595 (2009)
Libiao Zhang of Guangdong Entomological Institute in China and his colleagues have documented what may be the first case of regular fellatio in adult animals other than humans.
They report that female short-nosed fruit bats (Cynopterus sphinx) licked their mate's penis during 14 of 20 observed copulations. Matings that involved licking lasted significantly longer than those that did not.
Possible functions for this behaviour include stimulation to prolong copulation and assist fertilization; mate guarding; antibacterial effects; and the detection of chemicals assisting in mate choice.
The authors say their observations could suggest a possible adaptive benefit for the activity in this species.