There was no "allow" involved in my mono guy being mono and me being poly. There was only a terminology change when we learned the word.
My boyfriend has been mono his whole life. We met when he was 17. I refused to be his girlfriend early on saying I thought he needed to experience life and I wasn't interested in settling down blah blah blah.
He dated for a few years. But eventually we became lovers (I still refused a commitment). It was 17 years in that I finally agreed to a commitment that included him accepting that I am poly.
But-that's 17 years of loving me for who I am and accepting me for who I am before I agreed to make a commitment beyond him being my best friend with frequent benefits.
In all of those years, there was never a time I wasn't with someone else.
So-really, it was as simple as "this is who the woman I love is-and I love her." Nothing more and nothing less than simple acceptance.
"Love As Thou Wilt"