First, congratulations! Someone you know is getting married, and that’s a significant and happy thing. It’s quite likely this wedding couldn’t have happened two months ago, and it almost definitely couldn’t have happened ten years ago, which probably makes it even more meaningful for the people involved.
Given this newness, it’s likely you have some questions. Many social institutions are still figuring out how to recognize same-sex marriages and married couples. The laws are changing all the time; in the United States, the most recent significant change happened less than two months ago when the Supreme Court ruled that states must “license a marriage between two people of the same sex” and “recognize a marriage between two people of the same sex when their marriage was lawfully licensed and performed out-of-State.”* Since this ruling, approximately 40% of Americans still opposed this legalization.** Religious bodies are making new decisions as to whether they will bless these unions, and whether they will treat them equally with those of opposite-sex couples.

The cover of the New York Times from June 27, the day after the Supreme Court ruling, showcasing a dozen happy couples.
The questions you have probably depend a lot on your role in this wedding. You may have just received an invitation in the mail from a friend or relative. You may have been invited to be an attendant (i.e. bridesmaid, groomsman, usher, etc.). You may have been asked to officiate, to take photos, to bake a cake. Someone may have just proposed to you—in which case, even more congratulations! It turns out, in many ways the questions and the answers are the same as they would be in the case of an opposite-sex wedding. A wedding is a wedding, and most of them look alike in most ways. The only thing you can count on at a same-sex wedding is that there will be either two brides or two grooms and at the end of it they will consider themselves married to each other.
It turns out, a lot of stuff traditional etiquette or wedding planning books might tell you about weddings is gender-specific. Things done for the groom, by the groom’s family, with the bride, by her father, etc. Everything from who proposes marriage to who takes who’s last name has gender-specific traditions. And while anyone can choose to be walked down an aisle by or dance with a parent, everything needs to be planned and paid for by someone. These weddings can take more active thought and decision than opposite-sex weddings, because there isn’t a tradition to fall back on.
There are a few categories of things to consider if you have some responsibility for putting on one of these weddings:
The Law
Here in Pennsylvania, marriages between couples of the same sex have been legal since a district court ruling in May 2014. As of July 2015, marriage is legal throughout the United States regardless of the gender of the couple involved. This applies in all 50 states and the territories (with the possible exception of American Samoa). Federally recognized Native American tribes operate under separate jurisdiction, and can still decide individually whether to recognize and/or perform same-sex marriages. Currently, twenty other nations also allow these marriages to be performed legally. If the wedding in question is being held outside the country, the legal logistics may be different than at home.
However, there are places within the country where the law is not being applied. Couples in some counties have been refused licenses, and some state government officials have been encouraging this refusal. Additionally, while marriage is legal, in many parts of the country discrimination based on sexual orientation is also legal. This can make it more difficult for to arrange vendors for things like locations, flowers, cakes, photography, etc. When contracting for these sorts of services, it is often good to ask what experience they’ve had with same-sex weddings, and if they (and their staff) are comfortable with them. While there is value in fighting for your rights, there is also value in feeling supported and appreciated by everyone involved in your wedding day.
Religion
Many people getting married (even those not actively involved in a congregation) find themselves wanting a religious presence within the ceremony. As Episcopal Bishop Gene Robinson says, the state licenses unions; the church blesses them. That sense of blessing may manifest by holding the ceremony within a church, having it performed by a religious official, and/or having passages from sacred texts read. If one or both partners are members of a religious group, it is likely one where they feel accepted and welcomed. However, sectarian restrictions may prevent even a supportive religious official from officiating at a same-sex wedding ceremony, or holding one within their building.
While sects may have policies regarding entering, performing, or sanctioning same-sex weddings, neither policy nor doctrine address other kinds of participation such as attending or performing contracted services. No couple sends invitations to a wedding intending to cause people to sin, or even to witness sinning. They desire their guests to love, and witness loving.
Etiquette and Protocol
Many parts of weddings are dictated by tradition—cultural, ethnic, religious, and more. Weddings are rituals, and have meaning in part because they have so many familiar elements within them. Unfortunately for same-sex couples, many of these traditions are specific to one or the other gender. Adapting to a couple’s needs may be as simple as changing language (e.g. best men, groomswoman, couple’s shower), or as emotionally fraught as deciding whether one or both partners will change their last name.
Same-sex couples are older, on average, than opposite-sex couples when they get married. This may change in the future as the couples who were only waiting for marriage legalization take advantage of new opportunities. Nevertheless, because they are older, they are more likely to have established financial independence from their parents. This means that they are more likely to fund and host the event themselves, and may have less familial pressure as to the wedding’s specifics.
As anyone not a bride or groom, etiquette is pretty much identical to that at any other wedding. You are a part of this because somebody cares about you and believes you care about them. They think you and they would be happier if you showed up. If there are specifics you are unsure about (i.e. how to address the couple after they are married, what to wear, whether it is appropriate to bring children, what sort of gift to bring), ask!

Or, just wear your best top hat and rainbow unicorn horn. Always in style.
Want to know more about planning a same-sex wedding? About the process of legalization in America? About religious attitudes towards same-sex marriages? The library has some great resources for all of these!
Planning Guides
The Essential Guide to Gay and Lesbian Weddings
The Gay Couple’s Guide to Wedding Planning
The Lesbian Couple’s Guide to Wedding Planning
The New Gay Wedding: A practical primer for brides and grooms, their families and guests
Gay and Lesbian Weddings: Planning the Perfect Same-Sex Ceremony
Church and State
Blessings Same-Sex Unions: The perils of queer romance and the confusions of a Christian nation
The Bible’s Yes to Same-Sex Marriage: An evangelical’s change of heart
God Believes in Love: Straight talk about gay marriage
When Gay People Get Married: What happens when societies legalize same-sex marriage
Speak Now: Marriage equality on trial
From the Closet to the Altar: Courts, backlash and the struggle for same-sex marriage
Same-Sex Marriage in the United States: The road to the Supreme Court
Essays, History, and Etiquette
Here Come the Brides! Reflections on lesbian love and marriage
Same-Sex Marriage: The personal and political
Charity and Sylvia: A same-sex marriage in early America
Outlaw Marriages: The hidden histories of fifteen extraordinary same-sex couples
Steve Petrow’s Complete Gay & Lesbian Manners: The definitive guide to LGBT life
-Bonnie T.