I call it "monoamorous" rather than "monogamous" because while the partner wants to love only one at a time, they are willing to be in a relationship structure that is not monogamous shape.
There's isn't any "allow" -- there is "willing to be in relationship."
The first time around? That was how it was. I was not exclusive and stated as much up front. He knew what he was getting. So did BF2. They were there because they wanted to be with me and this is what it was here.
I married BF1. And today? It would be the same. The agreement has always been "Give me the heads up, we'll talk and see what we see."
Boundaries and agreements would need some revision to handle things like "time management" and more if still willing to be in relationship with me.
But he knows what he has here. He loves me like I am. I love him how he is.