'Grimm' Stars -- Commit Cardinal Sin in Naked Bike Ride

Fun Fact -- 2 studs from the show "Grimm" did a naked bike ride this weekend!
Not Fun Fact -- THEY DIDN'T ACTUALLY GET NAKED!!!

It all went down in Portland, OR on Saturday night ... when David Giuntoli and Sasha Roiz joined roughly 10,000 riders for the World Naked Bike Ride ... a jiggly protest against the oil consuming "car culture."

But when it came to dropping trou, David and Sasha totally wussed out -- instead of going bare-ass with their fellow bikers, the two actors only ditched their tops. Laaaaaaaame.

After the race, Giuntoli went to Twitter, where he blamed his prudish ways on "the trapping of celebrity" -- explaining, "We wore pants and shorts respectively. #keptourjobs"

So, the lesson here ...