Ugh that evolution/nicca comparison.
First, it's a straw man. "I've talked to liberals who said this, but now other liberals are saying something else. Since I can assign you other people's opinions via the Transitive Property of Political Bullshitting, doesn't that make you a hypocrite?"
Second, the liberal who drew that line would be wrong. Denying badly needed support to those who can least afford it because of what the guy says about evolution? As much as creationism offends my effete coastal sensibilities, that's the definition of the privileged selfishness that Loomis was talking about.
Third, it's pretty much cost-free for a Democrat to draw that line in the sand, because they know there's a 99.9% chance they'll never have to think about crossing it. A liberal in 2012 saying they won't vote for a racist, homophobic, young Earther is like a vegan "boycotting" Chik Fil-A. Context makes all the difference, and when your chosen issue is one where you're opposing a deeply entrenched consensus, then you are making a decision to abstain from mainstream electoral politics over the long haul, which is a pretty big f'n deal when you're reckoning with how to change policy.
What I haven't heard in all this discussion is a plausible alternative route for activists. "I'm not voting for Obama, cause he does Bad Things." So instead, you're going to do X, which will lead to Y through the mechanism Z. Please solve for the variables, specify any exogenous parameters, and try to stay within the historical data.
