I'm still hoping Cajun will sign up soon, he said he would. But in the meantime I can at least give you the idea that he came up with when it came to my exploring poly and the possibility of having sex with someone else.
Basic concept is if his pants don't have a problem, why should he have a problem. More detailed concept being, if nothing is changing between he and I in our sex life, or even if it is getting better and more frequent (which it was) then why should he have a problem.
If having a sub, helps him to be a more loving partner to you, and is not, in reality, physically or emotionally taking away from your relationship, then maybe it's more of a self-esteem problem. As you said, "If I say yes... you can't bring the person around me... I dont think I could handle the fact of seeing the girl your having sex that isnt me... I'm going to start second guessing everything about me. My looks, my weight, everything." That is all self-esteem, and although poly people can have as much self-esteem issues as mono people, it can be dealt with.
But I also agree with the others, don't change yourself to fit his mold if you won't be happy in it. Although the self-esteem build up would help all around.
Good luck, hope this helped and wasn't too off the wall.
Life is about the journey and not the destination,
so what better way to know life
than to wander all the roads and paths set before you.