Perhaps you should consider that very few of your actual target viewing audience (adults) browse the Internet with "Parental Control" turned on. For that matter, I'm sure the majority of minors don't use it either. ;-)
I don't think I'd want a young child freely perusing the Free Republic either. There is plenty of adult-oriented material on here, if you think about it. So it may be perfectly reasonable to filter your site, as well as this site. (Don't think I'm picking on political sites, I'd say almost any site where the "unwashed masses" can post freely is a candidate. Parents can always exempt your site if they feel it's appropriate to do so.
What I'd do is this: contact Symantec and get an explanation. If they base it on keywords, and so on I'd go find objectionable material on DU and other sites that aren't blocked, and request that they also be blocked, if that's important to you. If Symantec won't comply, you may always sue.
Hope that was food for thought...
Forgot to close a paren:
(Don't think I'm picking on political sites, I'd say almost any site where the "unwashed masses" can post freely is a candidate.)