Yikes! That's a bummer.

I've more often seen it from people who don't think things through terribly well.
Thing is, I think the label may be warranted, if you're really talking about someone who is indeed openly hostile towards whatever your cause or belief is. Christopher Hitchens, for instance, could probably rightly be called an anti-theist, while Valerie Solanas was indeed rabidly anti-male... yet not every atheist is an anti-theist, and not every feminist is anti-male.
I guess I wouldn't see myself automatically assuming that someone who holds a position differing from mine is anti-whatever, until I've talked to them for awhile and found out that yes, they actually are openly hostile towards whatever position it is (and towards me for holding it). That can happen. But not all the time, every time, automatically.
I dunno. I don't like assumptions.