Interestingly, my younger sis is not open minded at all. She thinks that everybody should get married before they have children and be monogamous. She has extremely strong views about the 'right' way to do relationships.
I have spoken to her about open relationships and having more than one partner. I'm not doing any of that right now but I very much value my sister's input in my life and like to know she's there to talk to should I need to.
And she was extremely interested in a non-judgemental way. Thought it sounded like a grand idea.
I was kind of fascinated because she has such strong, fixed views.