Liberace's Ex-Lover Scott Thorson -- Matt Damon Is a Gay Sex Pro

Matt Damon's gay sex skills are OFF THE CHARTS ... at least in his new Liberace movie "Behind the Candelabra" ... so says Liberace's real ex-lover Scott Thorson.

Thorson was out in NYC yesterday when we asked about the famous sex scene in the movie -- when Matt unleashes the fury on a buck-naked Michael Douglas -- and Thorson replied, "Matt sure can pound!!!"

Thorson says he was pleased as punch at the way Matt portrayed him in the movie -- but there was some collateral damage ... Thorson says, "I bet you Catherine [Zeta-Jones] is a little jealous."