I'm sorry you were caught by surprise and brought to tears.
I'm with everyone else who says tell FM to mind their own business. And I doubt treading lightly will get the message across adequately.
Of course, out of respect for my mother I'm abiding by an agreement I made with her when I was 19 to not come out to my family as Bi until my grandparents were gone. There's a lot of family I will NOT mind cutting myself off from so I'll probably be grinning when I come out as Bi AND Poly. So I may not be the best person to listen to in this instance.