Don't Say: "Which one of you is the bride?"
It's as simple as this: If it's a same-sex wedding, you'll see two brides or two grooms. If it's an opposite-sex wedding, you'll see one of each. And don't assume the couple is choosing to refer to themselves as brides or grooms. They may skip the terminology altogether.
Don't Say: "So, like, is this a real wedding?"
Of course it's a real wedding -- it's two people making a lifelong promise to one another. Whether your particular state happens to call it a "civil union," a "marriage" or nothing at all is beside the point. (And if you're curious about gay marriage laws, look it up!)
Don't Say: "When or how are you going to have kids?"
Unless it comes up in conversation, leave this question alone. It's not an appropriate question for any couple -- gay or straight -- on their wedding day. (And really, it's none of your business!)
Don't Say: "Oh, why isn't so-and-so family member here?"
A wedding is a time to celebrate and enjoy the company of those excited and affirming of the couple's relationship, and bringing up missing relatives who may or may not be in favor of the union could be hurtful.
Don't Say: "Thank you so much for the invite. I'm so excited to get to go to a real gay wedding!!!"
Even though you very well may be excited, absurdly gay-positive comments like this can be polarizing.
Don't Say: "I'm guessing the DJ will be playing nothing but show tunes tonight!"
Avoid applying stereotypes like this one. (Think of it this way: Would you say this at a straight wedding?)
Don't Say: "I bet you feel lucky to live in NYC -- it's too bad about all those gay couples in the other 44 states."
It's a wedding! Just like it's a bad idea to bring up unsupportive family or friends, skip "downer" comments like this one.
Don't Say: "What are your bridesmaids going to wear?"
It's not safe to assume that there will be traditional bridesmaids and groomsmen. The couple's closest VIPs might sit toward the front of the ceremony, or the tradition may have been nixed altogether.
Don't Say: "I wonder who will walk down the aisle."
Forget about the traditional male-female roles you're used to seeing at a wedding. Every couple (gay or straight) is different. Some walk down the aisle with a family member; some walk down the aisle together.
Don't Say: "Congratulations! I'm so supportive of your sexual preference."
Consider this: You wouldn't congratulate a straight couple on their engagement or wedding and then call out their "sexual preference" in the same breath. It's just as strange a thing to say to a gay couple.
Special thanks to Bernadette Smith, GayWeddingInstitute.com, and Alison Zack, MyBigFatGayWedding.com