I don't understand how legal and financial security pertains solely to mff relationships when there are no automatic legal or financial rights given to any non mf relationships (outside or recent fluctuation in the mm ff department). Are you against all polyamourous relationships due to this issues? Hows does mff differ?
There a certainly legal steps that can be taken to grant an individual similar rights as a married partner ie power of attorney, wills, and domestic living agreements. Financial arrangement's can be taken as well.
I get the feeling you think such a situation does not permit equality amongst the participants. Your words portray a second wife as someone who is being used or getting the short end of the stick. Where do these perceptions come from? Do you have or know someone with experience in a relationship like this?
We do not seek to harm or undervalue anyone. We want to find a partner with which we can grow as beings and better each other. We are not seeking a utilitarian or religious union. We are seeking a loving relationship.