Non-reproductive Same-sex Behavior in Animals
Source for all images: Bruce Bagemihl, Biological Exuberance, St. Martin's Press, 1999.

Two male giraffes, courting behavior

Two female koalas

Two male black swans, courting behavior. 

Two male swans may form a "trio" with one female, mating in all pairwise combinations.

Two female dolphins.
Dolphins of both sexes show exceptionally creative play.