Did Obama sacrifice huge swaths of swing voters? Probably not. The polls have shifted to net favorable on the question of same-sex marriage and independents are unlikely to punish Obama for a policy position consistent with everything else he's done on gay issues. Social conservatives will, of course, be aggrieved. But they were already.
Also, consider this on the gay-marriage pronouncements: Biden was first, Obama second. That the two happily reversed the common order of things ought to tell you something about their relationship (solid), their understanding of tactics (advanced), and how they can play Washington's chattering class for fools (easily).
Biden is central to White House decision-making on foreign policy and is Obama's best and most trusted surrogate when it comes to conveying administration economic policy to white working-class voters.
To summarize, Biden's relationship with Obama is sustaining and central to current White House thinking on politics and policy. Biden is a team player with no presidential ambitions—a vital component to a productive working relationship with the president. Obama learned this lesson from President George W. Bush. Pick a vice president who won't spend half his day scheming to succeed you. That way they can do more work for you.
Biden's stout loyalty conveys to political grunt work, too. Biden is the fundraiser-in-chief for House and Senate Democrats (ask them, they'll tell you in grateful detail) and therefore gets the rubberiest of the rubber chicken gigs. When Obama is feted at big-dollar fundraisers in Hollywood and Manhattan, Biden is pocket change in Hibernia and Manhattan, Kansas.
In fact, the seed for Biden's personal transformation on gay marriage was planted, according to Biden, at a fundraiser hosted by a married gay couple.
One last thing to consider in the Biden and Obama relationship: It shares a common denominator in the form of Bill Clinton and top staff who worked for Clinton. Clinton and Biden have always been allies and forged deep bonds on issues like violence against women, community policing, and gun control. The former president is now raising money for Obama, playing a central role in campaign videos, and advising Obama on his campaign.
That's Clinton's outside work. His devotees are doing a lot of inside work. Biden's chief of staff, Bruce Reed, was Clinton's domestic-policy adviser. Steve Ricchetti, now a counselor to Biden, was one of Clinton's deputy chiefs of staff. Obama's current chief of staff, Jacob Lew, was Clinton's budget director.
There are more, but the point is this: Much of what Obama did on gay rights before Wednesday's announcement was to undo or undercut Clinton-era policies, namely "don't ask, don't tell" and the Defense of Marriage Act. Reed and Ricchetti especially remember the grueling politics of both issues (gays in the military engulfed the earlier days of the Clinton presidency).