People who are incapable of understanding standard, real-world social cues and graces ("robots") lend themselves towards curious perversions and the groups that delight in those same perversions. There's always a primitive need for socialising, and when you aren't able to cooperate with real people, you'll find a group that you are comfortable with, that hopefully shares a lot of similar traits and interests as you.
When a community is built around the obsessive desire to create and roleplay characters that go beyond the normal, particularly those based around the wildlife that these robots seem to enjoy moreso that humanity itself, it becomes attractive those kinds of people. Moreover, as it starts attracting some "autistic" people, others see this a comforting fact that they'll be including too, without fear of reprisal.
It's basically a vicious cycle, where the stronger numbers that this community creates, the more degenerates who see it as a valid "escape route" for the problems in their own life. All I'm saying is that people get on my goat for liking to drink, but at least my drinking never involved masturbating to the image a 6-foot dog.
EDIT: I'm not saying that normal people can't appreciate the art and want to participate in that way, but I simply believe it's more than coincidence that a lot of the people we see who brand themselves as "furries" have social issues and are generally some of the less-well-received users of these forums.
EDIT2: Apparently some people needed further clarification; I don't just mean generally awkward or shy people. I'm talking about the absolute bottom of the barrel; those far, far removed from the real world and are completely incapable of sympathising/empathising with anything a normal person feels.