The vegetarian/vegan fringe can appear to be crazy. I'm pretty sure that PETA's philosophy is that any publicity is good publicity. Since veg*ns are a minority, I could see the fringe influencing many people's view of veg*ns, since they aren't always likely to take the time to know a veg*n.
Feminism has a crazy fringe that's just wrong. But it used to be that many people identified with feminism, which probably provided a buffer against the crazies. Now with the so-called "third wave" feminism lacking a core doctrine, feminism is becoming like spirituality for Americans - they believe in something, but it's not organized. Still, it seems to provide a buffer, at least for the time being. I could digress with this discussion, since it's one of the areas that fascinates me, but I probably should not.
As for civil rights is something that most people say they are for, so once again, the crazy fringe (perhaps the separatists would be a good example?) tend to be marginalized by people's own beliefs. Environmentalism is the same way - we may not agree with ecoterrorists, but it's not like we're going to start pouring used motor oil down drains either.
To properly marginalize a group, they need to be a minority, ideally a small minority. The group should be based with, or closely associated with an ideology that's somewhat flexible, so that the most extreme members can interpret it their own way. And, ideally, members of the group should be relatively unknown to much of the general population at large.