From the Boston Online Gay dictionary: " "Woof" in gay terms is the canonical gay bear sound of approbation. It's not a dog's 'woof', rather the breathy 'wuf' a bear makes when showing interest.
For those coming up to speed on Gay Subcultures 101, "Bears" (& Cubs, Muscle Bears, Daddy Bears, Polar Bears, Wolves, Otters, and other assorted furry menagerie) are the big hairy friendly lugs of the gay community. They range in size and shape but are typically identified by their Paul Bunyan attire and hirsute face.
There are Bear Clubs ("Mass Bay Bears") for Bears & fellas who are into Bears to get together to socialize. Bear Magazines, offering everything from porn ("Bear", 'American Bear", "Euro Bear", "Grizzly", etc.) to lifestyle tips ("A Bears Life"). Bear websites ("Bear411") for online congregating. Bear Events ("Out of Hibernation", "Int'l Bear Rendezvous", "Bear Trek"), and of course Bear Bars ("The Alley Bar"). There's even an annual "Mr. New England Bear" title holder.
The phrase "woof" and term "Bear" are widely recognized in the popular culture these days with references popping up everywhere from "The Simpsons" to "The New Republic". Indeed "Bear" has gone internationial with "Bear", "Orsi", "Orso" etc. groups organizing & fraternizing around the globe."