Teaching it is okay to steal if the person you are stealing from is rich. Mr. Hood poached the King's deer. The green wood itself is a symbol of virility and the references to arrows, quivers and swords make it clear, too. So his proof is green wood. Maybe Professor Knight doesn't get into the woods much but green is the predominate color in a forest.
Most trees are green.
As for the references to arrows, quivers and swords, those were the weapons of the day. They didn't have any AK47's in the 13th and 14th centuries. If the professor finds erotic imagery in arrows, quivers, swords and green wood then I would recommend he get counselling. Not only is there no proof Robin Hood was gay, there is no proof he even existed. He most likely is a composite character created from the many rogues of Jolly Old England.