If I am with you, I am with you. Regardless of what genders I am attracted to. It does not “widen the pool of people I could find more attractive than you” it means, out of every single variation of gender that I am attracted to, you are the most attractive and desirable.

Asking me if I miss women while in a relationship with a man is the equivalent of asking me if I miss men while in a relationship with a man. Of course I don’t miss other people, I am in a relationship because it fulfills me.