The "Fear Factor" donkey semen challenge isn't only gross for the contestants ... it's also really bad PR for donkeys everywhere -- that's according to the preeminent donkey advocacy group.

TMZ broke the story ... FF producers are serving up giant steins full of donkey semen -- with a urine chaser -- to contestants on their next episode.

Like most people who read our post ... the American Donkey & Mule Society had a visceral reaction to the news. The ADMS President tells us they don't "condone use of the animals in such a manner."

She added, "We feel this is a negative event and was not well thought out by the producers."

For the record, we've spoken with other sources in the donkey community ... who tell us no one should try this stunt at home because the "long term effects of drinking donkey sperm are unknown."