bookbug, again, I'm not talking straight logic. But let me thread the needle of my thought process just a bit for you. It's not that I couldn't lose her in a "closed" relationship. My past experiences have proven my ability to become single. My fear comes from inviting someone into what feels like a safe space and having to trust that they won't steal her away. That they won't undermind me and our relationship. That they are enlightened and want us to succeed. It feels a bit like letting the wolf into the hen house. Again, I'm not saying that everything I'm feeling is me at my best or pure logic. But it is based on some of my personal experiences.
So a "closed" relationship feels safer because there's just one person to deal with. Of course life is always messier and the devil is in the details.