For "unreasonable demands" to be effective they have to have some credibility. That is the other side has to have reason to believe that the people making them can also make trouble, that they are not just ineffectual clowns. Part of that is a simple matter of support. But I think it is also a matter of whether the demands themselves seem simply absurd. There is no bargaining power at the moment in a demand that the means of production be turned over to the workers, for example.
[WS:] Do not forget to add legitimacy to this list. It is more important than the believability of threat. Terrorists can certainly make believable threats and some of their demands even make sense, but they totally lack legitimacy and hence are not taken seriously. Gay right s activists that you cite, otoh, may have fallen short on the credibility of threat dimension (what could they do? Attack the police with dildos? :)), but they were strong on the legitimacy dimension by virtue of associations with concepts already high on the legitimacy dimension: privacy, personal freedom, civil rights.
Wojtek