Q: I have an etiquette questions concerning the addressing of graduation announcements for homosexual couples who are legally married and homosexual couples who live together. I am a high school senior class advisor and have had this question posed to me in regard to to how to address the envelope. Do you address the envelope as Mr. and Mr. (or Mrs. and Mrs.)?
A: Your question is a good one and illustrates the ever changing nature of etiquette. Any unmarried couple living together is addressed by whatever title is appropriate but with the names placed on different lines. Two married men may be addressed as Mr. and Mr. Two married women may be addressed as Ms. and Ms. Two married women could also be addressed as Mrs. and Mrs.
If you are certain that one member of a same sex couple identifies as a certain gender and prefers the form of address that is usual for that gender you would be welcome to use the appropriate form. That is something that would be a known preference to friends and family and you could inquire politely which one someone prefers. Otherwise, you would err on the side of biological conservatism and stick with a gender description that matches the known sex of an individual.