Because it seems, animals are not allowed to be gay.
The book is about two male penguins who bring up a chick, and is based on a real life incident in which a zookeeper noticed two of his penguins, Roy and Silo, were trying to hatch a stone.
The authors Justin Richardson and Peter Parnell were inspired by an article about homosexual behaviour among animals that ran in the New York Times and in writing the book spent time at the zoo observing the penguins to make sure they got their facts right.
(You can read a review of the book here.)
The negative reaction to the book in the States was expected since, as Richardson says:.
... homosexuality and talking to children about sexuality have been highly politicised.Now the book is to be launched in the UK, and no (here I wave my little Union Jack) such outcry is anticipated.
(Cartoon nicked from the Illinois Review - sorry, couldn't resist it!)
The Viz finds a story about gay flamingos.