I don't see any point in joining a group I don't agree with and posting on it. It pretty much like a priest showing up in a bar and shouting that drinking is a sin. No one is going to stop drinking, the bar isn't going to shut down, and all you do is annoy the crap out of everyone. XD
I also don't post on stories that I don't agree with! Unless the stories belong to my friends and I feel like they wouldn't mind me saying something opposite. But even so, I don't make a habit of this either.
Live and let live! ;-)