Boxing superstar "Sugar" Shane Mosley says Trey Songz DID in fact bang his ex-girlfriend -- but not while they were together ... which is why he's got NO beef with the singer.

Mosley just issued a statement about reports that Songz was nailin' a stripper named Bella behind Sugar Shane's back ... saying it just ain't true -- but if it was "he would be knocked out like most anyone else who ever stepped to me."

Mosley explained, "Yeah she was a stripper, yeah I was stupid to date her, but she was single when she was f**king with Trey. We didn't live together so she can do what she wants."

Shane says he DOES have a problem with his ex-wife Jin -- who now stars on the Bravo reality show "Friends to Lovers" -- insinuating that SHE leaked the story to the media to humiliate him.