Fight for you? Not physically fight someone else, but fight to do anything to keep the two of you together or is your relationship just fish love.
Not just my S.O., but my FWBs will go to just about any lengths to keep me! :-)
After a couple of failures we have found our FWB group that works for us.
It has not been tested in the way you mention in over two years but I believe they would in the same way I would for them.