Recently in the State where I live (California) the ban was lifted for same sex marriages. So now (or for now how ever you want to put it) it's legal to get married to a person of the same sex.
Ellen DeGeneres and Portia de Rossi are one of the first celebrity couples to get married under the new law. There are lots of regular folks out there that will and are planning on doing the same thing.
So what does one do when invited to wedding ceremony of a couple who are of the same sex?
Etiquette Expert Opinion:
Regardless of your opinion or beliefs. Just wish the couple well.
Don't take this as your opportunity to rant about the law or their lifestyle (Whether yo are for or against it - ranting is annoying).
If you are invited and chose not to attend, just RSVP in the same manner you would any other wedding (check the little line that say "will not attend"). Don't make a big thing out of it.
If yo do attend keep in mind there are some folks who might be uncomfortable with the play by play report. Be fair to all parties, and considerate of who you're speaking to when sharing information.