would say that changing just the gender restriction is a good start and far less complicated as expanding marriage to more than two people would imply redesigning some of the legal structure that goes with it - inheritance without probate, child custody, divorce etc. Not insurmountable, but would require a more complex legal underpinning.

Unless you do away with the special status completely. ;-)

But now we've fleshed out the justification with elements that involve families and children. Could it be that the original reason for the special status was to do with families and children at a time when heterosexual relationships were perceived as "the" mechanism to achieve that?

If so, then rather than applying today's prejudices to modernize the injustice by defining another sort of acceptable relationship, thereby shifting the injustice towards other sorts of people who also want to pursue happiness but are less equipped to defend themselves, perhaps we need to recast the laws to deal more accurately with the rights and obligations of parents or caregivers as they relate to children. Actually, I do believe that such laws mostly already exist and apply equally whatever sort of parenting scenario is in play. Removing the final vestiges of historic prejudice seems a more valid next step than simply adding another beneficiary.