Data is scarce, but there is definitely a lot of homosexual behavior among giraffes
From what I can tell by other places this claim is made on the web, the common source is Bruce Bagemihl's famous book, Biological Exuberance (1999). On p. 34-5 it states:
Three different species exemplify some of the issues that are involved in just one measure of frequency, the proportion of activity that is homosexual. Observed versus actually occurring behaviors in Giraffes, seasonal variations in sexual activity in Mountain Sheep, and alternative standards of reference in Gray Herons complicate the calculation of homosexual frequency in each of these species. During an exhaustive study of Giraffes in the Arush and Tarangire National Parks of Tanzania, researchers recorded 17 homosexual mounts and 1 heterosexual mount in more than a year (and 3,200 hours) of observation. Thus, 94 percent of all observed mounting activity was same-sex. Does this reflect the actual proportion of homosexual activity in Giraffes? Certainly more than on heterosexual mating occurred during that time, since over 20 calves were born that year in one population alone. However, these populations did have relatively low birth rates, and heterosexual mating appeared to be genuinely rare. In addition, if heterosexual matings were missed by the observers, probably homosexual ones were as well.
In a footnote he gives the source and considers another study:
Giraffe (Pratt and Anderson 1985, 1982, 1979). In a study of another population of giraffes, only three mounts between males were recorded, but only 400 hours of observation were involved (Dagg and Foster 1976:124).
Can't view the whole bibliography entry on Google Books, but I could swing by the library tonight and find it if someone expresses interest.