According to Jared Diamond (noted evolutionary biologist), just trying to have sex with a chimpanzee would be, quite literally, the accomplishment of a lifetime.

"...About a week and a half ago in Los Angeles, there was a really tragic, awful event in which a couple who had brought up a pet chimpanzee went and visited their pet chimpanzee in an animal shelter with a couple of other chimpanzees that had been kept there for a long time. These were chimpanzees habituated to humans. And something went wrong. Two teenage male chimpanzees attacked the man, and the result was just awful. They pulled off his nose, and [attacked] his eyes. They chewed off every one of his fingers. They chewed out his buttocks. Okay?

... Chimpanzees are about the same size as humans, and genetically they're the species closest to humans. Despite that, there is no attested case of humans having sex with chimpanzees. It would be absolutely lethal. The man who did it would come off without his private parts!"