Yesterday the Massachusetts Supreme Court ruled that same-sex couples that want to marry in Massachusetts but are not residents of the commonwealth cannot do so if they come from a state that forbids such unions. This doesn't seem entirely outrageous to me at this point in the fight for equality in civil rights - marriage laws are generally the province of the states. However, there is a long tradition of people going to other states to take advantage of more liberal marriage (and divorce) laws, Nevada being the most prominent. Couples would run to Vegas or Reno (and still do) to avoid waiting times and blood tests and residency requirements. I believe Delaware was where East Coasters eloped to. (However, when they returned home, their marriages would be recognized in their home state, thanks to the Full Faith and Credit clause of the Constitution.)
Mitt Romney, however, doesn't want his state to turn into the Las Vegas of gay marriage, so he was happily trumpeting the triumph of what he sees as justice. But I have yet to hear a single logical reason why extending equal civil marriage rights to same-sex couples would undermine "traditional" marriage. In fact, here's an excellent argument that lays out why two flavors of marriage don't have to interfere with each other.