How do you address an invitation to a gay or lesbian couple?
The same way you would address it to any other unmarried couple or married couple with different last names. On the outer envelope of the invitation, the two names should be alphabetically listed on separate lines:
Mr. George Bellafante Mr. Thomas Jones
On the inside envelope, use titles and last names: Mr. Bellafante and Mr. Jones. Same with the place card.
If, however, they are married, you have a couple of options, depending on how formally you want to address the envelopes. Because you won't be saying Mr. and Mrs. for this invite, you'll probably want to address it this way:
Ms. Joan McDermott
Ms. Theresa Smith
This way, you're not saying Ms. and Ms., which would be awkward. Joan comes before Theresa alphabetically. Another option if you don't want to use titles or put the two on separate lines, since they are, in effect, married:
Joan McDermott and Theresa Smith
Wedding Invitations + Wedding Stationery