HBO's Big Love is a fascinating series (see a detailed recap of the current season at The House Next Door), in large part because the acting is so understated for a show with such a bizarre premise. The one aspect of the show that is the most difficult to accept is that a seemingly intelligent, self-confident, and "modern" woman like Barb (Jeanne Tripplehorn) would accept her husband's taking two more wives.
I don't think Barb is so modern, but I suspect that HBO and/or the producers are being cagy with her character. In last night's episode, she crashed a meeting of the Eagle Forum, founded by Phyllis Schlafly, and she seemed pretty comfortable with the group. I'm guessing that Barb, as well as Bill and Nicki, are strongly against abortion, gay rights, and the theory of evolution. In the context of the show, polygamy is as culturally conservative and as "pro-family" as you can get, and Barb's decision to follow "the principle" (albeit with recurring doubts) makes much more sense if you see her as a member of the religious right, complete with antipathy toward the government and disgust toward secular society.
I think Barb can be a right-wing Republican and still be a decent person with humane instincts (to a certain point, as is true of most of us). But HBO probably fears that conservative protagonists* would alienate its upscale, well-educated audience, so I doubt that Barb will be doing much political commentary.
*On The Sopranos, Carmela occasionally showed her Republican leanings -- defending George W. Bush in an exchange with daughter Meadow, for example -- but, of course, she was already a morally compromised character.