Clearly his statement supporting the rights of anyone to enter into a marriage with anyone else regardless of gender has no immediate impact on the law regarding marriage or civil unions.
So what is the real impact this has?
IMO, the biggest impact is that friends and families around the globe (not just in America) will be discussing this over the next few days and weeks openly. In many families and between many friends, this will be a topic that has never been discussed. Or if previously discussed, this will either challenge the issue for reconsideration or affirm your previous beliefs.
Families are now allowed to discuss this at the dinner table. This will probably result in a lot of hurt feelings and examination of the fundamental issues at stake.
This global discussion will occur in the US, in Europe, in China, in Russia, heck, probably in Afghanistan too. This is going to be a global topic.
So regardless of whether you think this is going to help or hurt Obama politically in his re-election bid (I personally think it will help him), the bigger effect will be a global re-examination of peoples viewpoints on the topic around the dinner tables of the world.
This can only be a good thing for equality.
edited the title to reflect the content after reading marykk's diary about the dinner table conversation.