Exotic Extended Marriage: The only thing to note about these, really, is that they’re not all that exotic, what with the laws on marital contracts letting you hack together pretty much any topology you like. Still the minority choice, admittedly, but not so much so that everyone isn’t certain to know at least one person in one.
Which leads to the RSVP sections of invitations having a section ‘There will be [ ] sophonts attending in response to this invitation’
Possibly with attachments for species and etc.