Public opinion research tells us that individuals' partisan identity crystallizes in early adulthood and is resistant to change as we age. The same is generally true of our attitudes on political issues, albeit with more wiggle room. As the political environment changes and as our views and priorities change (I believe sociologists still use the term "life course effects" to reflect the evolution of outlook that comes with major life events like college, entering the workforce, marriage, parenthood, home ownership, retirement, and so on) we are more willing to change our tune. If the bottom suddenly falls out of the economy, we become a bit more favorable toward things like unemployment benefits. When we buy a home we start to care about property taxes. When we're retired or about to retire we become much more pro-Social Security/Medicare. We do, in short, shuffle our priorities and change our minds in a systematic way.
As long as they're not doing it at apparent random, it's never a sign of weakness for political figures (or ordinary people, for that matter) to change their opinions over time. Thoughtful people tend to do that. It's not a sign of intellectual weakness. I used to be in favor of capital punishment; as I learned more about, and gave more thought to, the issue I changed my mind. Please don't think less of me.
There are bonus points to be won for entertaining these changes of heart and mind when it is politically unpopular to do so. No one gets a round of applause for coming out against Jim Crow-era segregation in 2013. Once opposing segregation became overwhelmingly popular, taking that position skirted the fine line between Evolving Opinions and political opportunism. Coming out against segregation in the 1920s – now that would be worthy of a hat tip. That involved some risk. That meant accepting the risk of taking an unpopular stance.
As all of the data from the last 20 years show, there has been a dramatic change in Americans' attitudes toward gay marriage over time. It has gone from a political third rail (Remember when gay marriage referendums used to be put on ballots because the right knew that people would come to the polls just to vote against it?) to a rapidly moving bandwagon in an astonishingly short period of time. Whereas people used to be hesitant to speak out for it, people are becoming more hesitant to oppose it for fear of being perceived as backward, bigoted, and behind the times.
Politically, being pro-gay marriage is a relatively easy thing to do now. For Democrats, it's downright popular. For Republicans, it has majority support among the younger generations and the Unspeakable is being spoken now even among the older ones. Gays now serve openly in bastions of macho-hetero symbolism like the military, major corporations, all levels of government, and throughout the media. Even in the bone-headed culture of professional sports, gay-bashing is met with immediate rebuke these days (remember Chris Culliver, or Kobe Bryant scolding homophobic fans on Twitter?)
All of this is a lead-up to the question of why Hillary Clinton came out in support of gay marriage in March of 2013, and whether this is supposed to impress anyone. For all her strengths, HRC has been remarkably risk-averse in her political career and has come up woefully short on this issue time and again. It's the reason that many people, myself included, have criticized her as a political opportunist, a party insider who advances into new territory only after it has been made safe by others. It's not exactly a heroic move at this point, and it's difficult to believe that there was no point before now – just months before the Supreme Court may be about to resolve the issue and after almost every Democratic elected official of any note has already endorsed it – at which she could muster the courage to take a stand.
A key criticism from the left of both Clintons and their DLC-type followers within the party is and has been that they adopt issue positions based on popularity rather than taking positions and explaining why people should support them. What Hillary Clinton has done with the gay marriage issue is a perfect example of that. Don't get me wrong, I'm glad she has come around or "evolved" to what I believe is the correct position on the issue. That said, her unwillingness to support it before it was declared Politically Safe to do so is a good example of why she couldn't rally Democrats behind her in 2008 and will have trouble doing so in 2016. Fortune may not always favor the boldest, but it certainly does not favor the least bold.