So...I was told today that Facebook has now made a change to our relationship status options.
There is now an option for "Open relationship".
I changed mine to reflect that status just a few minutes ago.
Caveat: If you had it registered as "married" before, it will send a confirmation message to the person you said you are married to. Wasn't a big deal for me, but to some, it might be.
Just thought I would share.