You shouldn't out people before they are ready.
Mark knew his friend, Joe, and Joe's partner, Neil, want to come out to their family.
Mark wants to send out invitations for a celebration party for Joe and Neil.
Mark send out invitations for a mystery party to Joe's and Neil's families.
Mark sends out invitations to Joe's and Neil's coming out party to their families.
All families RSVP positively and Joe and Neil have a chance to talk to their families first.
Both families find out about their relationship from the invitations and RSVP saying they won't be coming.