The informant learned the following folk metaphor from a friend in college:
Gayer than a tree full of monkeys on nitrous oxide
The informant uses this figure of speech around friends who are gay and know that [hes] not gay-bashing.
The informant feels comfortable using this figure of speech around his gay friends because he considers that in general, gay people are really laid back about their orientation. He calls the folk metaphor kind of silly and over the top but not horribly offensive unless its used in an offensive tone.
If we take as a given that the folk metaphor is indeed silly, it is not surprising that it involves monkeys, which seem to be seen by Americans as fun or amusinga barrel of monkeys is defined on Urbandictionary, a website that allows users to define slang terms, as a standard of comparison for describing how much fun something is and Amazon.com sells joke books called Barrel of Monkeys Super Silly Joke Book and Cheeky Wee Monkey Joke Book. Nitrous Oxide, while generally used as an anesthetic, also has the side effect of causing temporary hysterical delirium and is nicknamed laughing gas, which would explain why it is being used as part of a silly scenario. In this case it seems safe to say that the folk metaphor is implying that gay people are a source of amusement.
The folk metaphor is obviously widespread; it is pictured here on a commercially produced T-shirt: http://www.zazzle.com/gayer_then_a_tree_full_of_monkeys_on_nitrous_oxide_tshirt-235612046927597732
It can also be found in Neil Gaiman and Terry Pratchetts novel Good Omens: The Nice and Accurate Prophecies of Agnes Nutter, Witch (151).
Sources:
Cusa, Nick. Barrel of Monkeys. Urban Dictionary. 25 April 2011 <http://www.urbandictionary.com/define.php?term=barrel+of+monkeys>.
Gaiman, Neil and Terry Pratchett. Good Omens: The Nice and Accurate Prophecies of Agnes Nutter, Witch. New York: Harper, 2007.
Nitrous Oxide. Britannica Concise Encyclopedia 2010. Encyclopædia Britannica. 25 April 2011 <http://www.answers.com/topic/nitrous-oxide>