Unfortunately what Obama did was not really a flip flop. He's supported gay rights in the past, he's been against an anti gay marriage amendment, he's supported civil unions with full rights, he's admitted his views on the subject were evolving, he's just been against gay marriage in specific. If someone like Santorum were to support gay marriage all of a sudden, now that'd be a flip flop. Then again I'm surprised Santorum hasn't publicly supported burning homosexuals at the stake.
Obama coming out in support of gay marriage would only be a flip flop if it was the opposite of his former position. However based on his words and actions of the past this is more of a natural progression. I don't think you know what a flip flop actually is.
Exactly, this is not a 'flip flop', it's a minor evolution on a detailed part of gay rights.
Now, if you want to ask if Obama has adjusted his position for politics - I think so.
But his position is nothing like the average Republican much less the greatest flip flopper in modern politics, Mitt Romney.
And Obama has the benefit of going from 'mostly right' to 'right' - being right an issue counts for more than the minor or major change on an issue. Give me a politician who changes his position to the right one over one who sticks to the wrong position any day, that's more important.
Obama has generally been pro-gay rights (so was Romney earlier, saying he was for more rights than Ted Kennedy). Obama held off on one; and now is more consistent supporting that one as well. Romney on the other hand has totally abandoned all his previous position for rights, now opposing all gay rights on the table, to the right of most Republicans, opposing not only gay marriage but civil unions, supporting his church that spend millions out of state to fight the end of discrimination, and opposing removing Don't Ask Don't Tell.
So Romney has gone from yelling how he's totally for gay rights and more than Ted Kennedy and how seriously he takes them as civil rights (when convenient) to a total reversal strongly anti-gay equality (when convenient), while Obama has gone from a long list of supporting gay rights with one exception to removing the one exception (when he'll alienate a lot of bigoted voters). Only a liar will say those are equivalent.