POEM - PLEASE HELP!!

Can anyone please tell me who wrote the poem Christopher quotes to Diana when they are about to be discovered by Robin? Christopher says "Owen wouldn't...." so I assume he means the poet; however the only Owen I know is Wilfred Owen, but all his poetry (wonderful as it is) is about the first world war.

Can anyone help as I really loved it and would love to memorise it. You will also save my sanity as it's really nagging at me that I've heard it somewhere before. Many thanks!!