My impression definitely is that fandom skews liberal. I can't speak to BSG much since I've never seen it, but I know for BtVS/Angel and even Firefly the "villains" are constructed in such a way that viewers are much more comfortable calling them Evil than they are in the political world of their own lives. (I could get off on a tangent about how the American Left has demonized the Bush Administration and often reflexively defends anything that Administration is opposing -- and opposes anything that Administration does.) Also, much media is very anti-authoritarian, which I think is a theme American liberals are very comfortable with. Additionally, that comfort can cause said liberals to gloss over the fact that the anti-authoritarian people are enacting their own authority/power in sometimes problematic ways. We are cultured to root for the underdog, and it's very easy for us to support ethically shady things when they are in utilized in opposition to something we view as reprehensible. (There is also the argument that Those In Power control the game so the Good Guys are forced to cheat in order to have Good ultimately triumph. Not an ethics compatible with Christianity I would argue, but one many people both secular and religious often don't problematize.)

Watching S7 BtVS for example, with its talk of war etc., I very much felt that the writers were making a statement about what was going on in the political context of the writers/viewers. It seemed like the writers wanted us to sympathize with Buffy, however, which I had difficulty getting my head around both because it was difficult to do so (cue lots of meta talk about how Buffy has never been trained to be a leader) and also because mapping the S7 conflict onto our own political landscape she often seemed to be a stand-in for figures I felt confident Joss disagreed with very strongly.