And Tango Makes Three by Justin Richardson & Peter Parnell

A picture book about two male penguins at the Central Park Zoo who raised a chick together. Based on a true story. Has appeared on the ALA's most challenged list more times than almost any other book in existence. 

At New York City's Central Park Zoo, two male penguins fall in love and start a family by taking turns sitting on an abandoned egg until it hatches.