The reaction to Eich is best seen through a lens of human group power dynamics, as it isn't logically consistent with some supposed ideals (civil discourse, etc.). Geeks have a harder time understanding this social acceptability aspect, especially within a given group or tribe. For this particular political question, a "no gay marriage" opinion has passed into the socially unacceptable in the Bay Area tech crowd.
I'm not saying it's right or wrong to react or ostracize like this, it's just the way crowds of people tend to work. And when tides shift in one direction, a desire by the winning side to celebrate, express power, and pick on the losing happens.
In 2008 it may have been a serious political question in California and the United States as a whole - and we totally can judge based on popular opinion - the difference is supporters narrow the judgment of social acceptability to Eich's narrower group of Bay Area techies. That's why saying it's "obviously wrong" or "there are no rational arguments" is a cop out - and the same denying civil rights argument can be applied to say abortion (denying women's right to choose! Murdering unborn infants!) or opposing affirmative action (racist against minorities! discrimination!).
The main difference? Public opinion rapidly moved against Prop 8 whereas for gun control or abortion it has been far more steady. [Even though over 20% of San Franciscans voted for Prop 8, the number among the educated tech crowd was likely far lower, and same among California as a whole]. Meanwhile, a movement like neo-Nazism is more universally reviled and not socially acceptable in 2014 US, although it may have been mainstream in 1938 Germany, and speaking out against it would leave you ostracized. This is where the "right side of history" aspect becomes interesting. Everything happens within the context of your time and your tribe, and this morality breaks down after these context changes.
Because the opinion on gay marriage is so universal within the tech crowd of the Bay Area, Eich doesn't really have a way out in this. (Look at the contribution numbers for employees of Google, Facebook, etc.) Meanwhile minority opinions on gun control, abortion, etc. are still socially acceptable but will still trigger suspicion. Ditto on economic issues and the entire anti-libertarian techie backlash, even though libertarianism continues to be weaker than liberalism. For another industry and group, look at Whole Foods' CEO writing an anti-Obamacare Op ed which triggered a backlash - the difference is that Eich said nothin, and most definitely continued to adopt gay-friendly policies at his workplace.
In medieval Europe, the adherence to your group (or broader society) was enforced through excommunication, in communist Russia, through purging and re-education camps, in Hollywood during the Red Scare, via McCarthyism and impossibility of finding a job. In Silicon Valley, is it by Internet campaigns and removal from high-ranking positions? What supposed Eich defenders are saying is - let's try not to be like that, no matter how righteous we believe our opinions to be.