Maybe they are afraid that the answer would be "a big bannana." That would be bad news on several levels. Although it might be the first gorilla joke.

There's so much potential for sleazy jokes and raunchy flippancy with a subject like this, it's almost impossible to know where to begin.

So let's stay serious, for a moment. It's obvious Deb Thompson didn't do very much research on gorilla sign language before launching into her suggestions on how other people should do their jobs. If she had, she would have learned that there's a serious amount of scientific dispute over whether gorillas even CAN communicate with sign language.

Judging by some of the examples cited by Cecil Adams in his "Straight Dope", all of Koko's (and other gorillas) success comes from how their scientists "translate" their answers.
http://www.straightdope.com/columns/030328.html

Let's not forget the self-promotion element as well. Koko is a major "industry" for the scientists studying her. They have every reason to inflate or even fabricate her successes far out of proportion. Judging by the transcripts at www.koko.org, Koko's handlers are experts at turning her every gesture into a "sign" which is contextually relevant.

Besides, the solution to Casey's problem is obvious. What does a male gorilla want in a mating partner? The same thing every other hetero male on the planet wants, regardless of species. They want a female who looks like (or is) Neve Campbell.