No it fucking isn't. Most true American lefties will only choose Liberal because they're the alternative to Republicans, assuming they decide to vote at all. Liberal party doesn't really hold a great deal of true left-wing values, or even true liberal values in the original sense of the word. It's the dilemma of a two-party system. The party professing to be on your side might be full of twats, but if you don't vote for them, it's "throwing your vote away".
Also, in my country, the Liberal party is the rightwingers. It's Labour who's more left-oriented. And I support the Greens, for what little good they do. Labour can go to hell, they way it's run at the moment.