Why wasn't Uncle Monty undressed in the bedroom scene?

Richard Griffins, being body conscience and embarrassed about his size, refused to shoot the scene naked, but promised he'd be able to convey the scenes intent without having to be naked. I heard that from either one of the docs about the movie, or from one anniversary Q & A's with the cast of the movie.

Well, he ‘undid’ his dressing gown so when he’s cornering ‘I’ his tackle is exposed, though we mercifully don’t have to see it.