I find this sentiment fascinating. Though that can be applied to almost any "you like x so your opinion doesn't matter" sentence (aside from certain things). I've had this happen to me twice recently and it's just...baffling.
To give a tad bit of context, this was on an unrelated fan group. Someone made a post about super heated opinions and arguments about characters in a game. I make a passing joke about how I have been blocked more in that group for simply having a different opinion on some characters than I have for the most heated views on just about every furry board I've frequented. New person comes in, pitching a fit about how dare people have opinions and complaining when "this wasn't the place for them" (this is still in a group where you can make threads, mind you). OP and I pointed out that he was being unreasonable and the only one bitching. He then becomes even more belligerent and pretty much outright stated he wouldn't bother talking with me because I've frequented furry boards and any point I've tried to make, he'd just throw a jab about my being a furry and leave it at that.

I've seen it done in other situations but it'd be different. Like...say someone is in to x genre of music and someone else is in to y genre of music. Person in to x genre can hold it against person in to y genre and say their opinion on music is invalid because they're in to y genre. But I've never seen something like "oh you're in to Z book series so obviously you lack any opion on anything" when still talking about music.

I'm not mad and being all "wahh fursecution". I'm just, baffled how someone can take something so trivial and then nit-pick the fuck out of everything you would think or just not bother with your opinion. I don't know, maybe my years of learning how to debate properly on the internet prevents me from thinking like this.