I looked up the sources mentioned above:
Pratt DM and VH Anderson. 1982. Population, distribution and behavior of giraffe in the Arusha National Park, Tanzania. Journal of Natural History 16 pp481-489
This article is about population data collected over a year of observation. It mentions there have been 22 births in that year, a low birth rate considering the population included 172 females. It says nothing about mating or homosexual behavior.
Pratt DM and VH Anderson. 1985. Giraffe social behavior. Journal of Natural History 19 pp771-781.
This article summarizes the authors' observations and statistics of social interaction in giraffes, collected in three separate national parks in Northen Tanzania, over about 3 years of total (non-consecutive) observations, more precisely 3264 hours of observation. They mention that they integrate findings from the '79 and the '82 articles in this summary article.
They say that sparring bouts, which usually involve "necking", when giraffes swing their necks and hit each other with them, sometimes included one male attempting to mount another. "We saw this eight times in the Arusha Park and eight times at Tarangire. In nine of these 16 events, the animal attempting to mount had his penis unsheathed. [...] In no case did a bull try to mount another male as large as himself. Intent observation and detailed recording of positions and movements failed to show that this act was an expression of dominance (which we initially surmised); we never saw any indications---e.g., behavioural, postural--of submission in bulls mounted."
The article goes on to discuss courtship and mating. It appears that it's very rare for giraffes to actually mate. The female is in heat only one day out of every two weeks. The males test females for readiness by sniffing their urine, but the female can choose who to give this data (by choosing to urinate or not when the male lowers his head to her rump). When a male giraffe senses that a female is in the right condition, they may proceed to courtship, which involved 1-2 days worth of following her closely and occasionally attempting to mount, which the female usually frustrates by just walking forward. When the female chooses to stand still, the mounting will proceed. The researchers note with some exasperation that over their 3 years of observation, they only saw one successful mounting, out of 46 courtship attempts and 304 urine-sniffing attempts. In light of the fact that in one of these years 22 calves were born as discussed in the previous article, they must have missed many - but of course they could also miss many homosexual mounting attempts.
Dagg, AI and Foester JB. 1976. The giraffe, its biology, behavior and ecology. New York, Van Nostrand Reinhold.
This book briefly mentions that homosexual mounting was frequent (no numbers) in one park where the number of females was unusually low, about 30%; in another park with approximate parity between the sexes, it was much more rare, and they only saw 3 attempts in 400 hours of observation.
"Necking" is not treated as homosexual behavior by any of these sources; they mention that necking is reserved only for males, and seems related to dominance relationships between males.
My conclusion from all this is that although it's arithmetically true that in 3 years of observation, the authors of the '85 source saw 16 homosexual mountings and just one mating, it might be misleading to summarize that as "94% of sexual activity is homosexual". First, the observation apparently missed the vast majority of matings, and probably of homosexual mounting as well. Second, the male giraffes seemed interested in much more coupling, but the menstrual cycle and female refusal combined to give them only incredibly rare opportunities.