Lets assume a companion says "oh I'm up for an open relationship" in act 1. As the story goes on, they fall in love with you. In act 2, they'll say "nevermind, I'm actually not up for that, I didn't expect to love you as much as I apparently do". To then have them suddenly be okay with it again in act 3 is inconsistent, even if they were okay with it in act 1. Unless they stops loving you as much. Which needs to be mentioned, implied, written down.
I think the flaw in your argument is the assumptions that loving relationships = monogamous ones. Poly relationships are loving relationships - that's why the latin and greek are smashed together poly-amory / multiple-love
So she loves you and is open to being with other people who will share nicely.
