The M.O. of the American liberal is to attack pig-headed bigotry whatever its source, and a lot of that bigotry tends to go in Islam's way. So while one can say that Islam is a relatively dangerous religion, a liberal will attach a greater weight to the dissension against Islam because in American society that dissension is in fact more prevalent and the greater immediate danger. (See for instance the hate crimes against Sikhs.) This is why there is no real confliction when a lib would defend Islam in one moment and then passionately dismantle Christianity the rest of the day.
Most Muslims in the world are too busy being practical human beings, struggling to raise families and make ends meet, to be the irrational, anti-democratic, mysoginistic goons their faiths would incline them to be. But that's a different story.