In our union of states, reciprocity is paramount. Each state recognizes educational degrees received in other states. Each state recognizes and accepts the validity of marriages in other states. So, when there are just 13 left that don't recognize gay marriage, the reciprocity of the union has to be preserved making same sex marriage universally acceptable and recognizable in all 50 states.
My concern is that, if gay marriage is now the law of the land, will any religious objection to it be voided? Will the refusal of preforming a gay marriage by a Church be overridden by what the Supreme Court just did? My guess is that we will have nothing but one court battle after another in which gays fight any and all objections to their marriages. Mark my words.
Supreme Court rules in favor of same-sex marriage nationwide: http://www.cnn.com/2015/06/26/politics/supreme-court-same-sex-marriage-ruling/