Some of my most conservative friends, if I asked them about many large social programs they would not be in favor of removing them completely (although that would be something interesting to hear a real debate about). Most of them are pro-national defense and not against some infrastructure and social safety nets. However, most of the reform policies that they would be in favor of have been deeply and rigorously villainized by the liberal coalitions.
In other words, unlike many self-described libertarians, they are not wholly against government programs but believe they are bloated, unwieldy, stupid, and corrupt--their sheer size and scope invites corruption. (In all fairness, many self-described libertarians agree.)
I've written recently about the libertarians giving themselves a liberal makeover (what could be called Miss America libertarianism, to riff on a cute phrase).
Now we have Rob Portman "coming around" on using the state's power to make everyone acknowledge same-sex unions as part of the millenia-old, pre-state institution of marriage. Whether because he doesn't want to risk hurting his son's feelings or because it gives him an excellent opportunity to get that post-Romney political makeover that some politicians are looking for, or both, we may not ever know. He has a much more personal reason than either Biden or Obama did when key culture war issues provided a convenient distraction from questions of fiscal leadership. How timely! (Portman's conversion may be timely too.)
Matthew Yglesias, and presumably many more progressives, thinks this is narcissism. How many liberal causes you must take up before you are no longer guilty of narcissism? Yglesias doesn't say exactly although he throws some examples out there right from the big liberal playbook. Yglesias apparently worships at the εκκλησία of liberalism.
Now, elsewhere liberals are likely to hold that principled consistency is absolutism and black-and-white thinking--usually just when applied to conservative causes, oddly enough. When one is badmouthing the Tea Party is convenient to juxtapose them with "reasonable" (i.e. more liberal) Republicans. (McCain almost seemed to win the GOP nomination more for his popularity with liberals than with conservatives, that "maverick.") Principled consistency in sticking to the progressive platform is not absolutism for liberals-- just goodness and mercy.
There is no slippery slope!!! No!!! You can avoid the slippery slope by being as selectively compassionate as you wish. However, some of us like to care about everybody. And we need everybody's money to care for everybody... (Not all the narcissism is on the conservative side.)
It's all part of the progressive narrative.