I've wondered about this, and as it relates to weddings I wanted to gather some opinions. I know that a formal invitation or letter requires formal addressing, and traditional etiquette dictates that the man's name goes first. But I have two questions:
1) When you address invites to a same sex couple with different last names, how do you determine which name goes first? Alphabetical? Call and ask? The one you've known longer or are related to?
2) What if the couple is male/female but you know the woman and not the man? Example, my female cousin has a male domestic partner but I have never met him, of course he gets named on any invitation or letter, but is it insensitive to my cousin if I put his name first just because he is a man? Or is it bad etiquette if I put her name first since I know technically it should be his?
I love learning about etiquette, but etiquette evolves, and I'd love to hear what you all personally choose to do in these situations.