Looking for adaptive benefits in all the wrong places

We are posting this Research Highlight from today's Nature magazine in total, because it's short and because, well, it deserves to be read in full. 

Research Highlights

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.

Two things: 1, if you go to the paper in PLoS One, you'll note that there are no pictures. 

And 2, adaptive benefit?  Why does there have to be an adaptive benefit??? Can't it just be fun?