Bit of hyperbole there to make fun of those who do.
Why is it insecure to keep someone around who's already proven they deserve a place in your life? (and who's to say they're not platonic at this point?). Lots of people have dated their friends on the way to getting to be friends with them. Heck, if a we had to cut out every friend we'd taken a detour down that road with, some of us wouldn't have any friends around at all. lol
That's a big price to pay for someone who just came along, especially when they haven't done anything to confirm whatever it is that makes this a problem for you. Nobody wants their partner choosing their friends (and let's face it: chosen family) for them.
I guess if I could give you any advice here it would be to take a step back, and ask yourself is this is something you can learn from, or if it's an irreconcilable difference and move on.Unless you have a child with that person why keep them around? When you keep your past in your present you leave no room for your future to arrive.