Myth: Sci-fi fans are chronically single, underemployed, and still live in mom's basement.
Not true. Any such broad and demeaning stereotype should be treated as suspect. Fans out of high-school move out just like everyone else. I don't know where this stereotype comes from but the fan groups I'm familiar with are populated by a wide range of professions, none of whom live with mom and dad. These people are either married, engaged, or dating. The single guys are the exception rather than the rule.
Myth: Sci-fi fans are predominantly male.
I believed this one for a long time myself. But once I started attending larger events, I found that every mulit-genre convention I've attended has been about 50/50 genderwise. Granted some of the smaller comic book events have skewed male, but every Anime, Harry Potter, Star Wars event I've been to has been very gender balanced.
Myth: People who wear costumes to Science Fiction conventions are arrested, overweight, adolescents.
Wow. That's a low blow. Costuming is a science fiction tradition that dates back to the earliest conventions. I know costumers who are doctors, lawyers, engineers, and even US Marines and I wouldn't call any of them arrested or overweight. I think number of the overweight costumers is the same as the percentage of overweight fans in general, and probably society as a whole. I don't know where this level of resentment comes from, but I've heard some variation on it from several people.
Myth: Furry Fandom is made up of perverts.
Come on, now you are just being mean. Don't let CSI form your opinions for you. Besides, the fans with the fursuits make up a small portion of those in furry fandom. Many are writers and artists that work with predominantly anthropomorphic characters. Thinking someone in a fursuit is into atypical sex is like thinking someone in a leather jacket is a bondage freak. Grow up and get to know people before you make calls like that.