Obama included the caveat that this was his personal belief and he thinks states should have the right to set their own policies. According to Stateline.org, 41 states have explicitly forbidden same-sex marriage, seven (including Washington, D.C.) allow it and three may allow it depending on the results of current legal challenges.
Mitt Romney restated his position that "marriage is a relationship between a man and a woman" after news of Obama's statement broke, according to ABC News.
Governing has collected online reactions from our Twitter feed, staffers' personal feeds and the #SSM hashtag in the Storify below. Let us know any we might have missed in the comments; we will update this Storify over the next few days.