I told my husband the same thing, except, I told him that if he ever cheated on me I'd go get the ***** and turn her out (meaning, she would no longer want what he has, if you get me). It was said in fun at the time but he knows me well enough to know that if I ever wanted to put action behind those words, I could do it easily and without remorse.
Your husband wants an open marriage anyway but he's respecting you and marriage in the traditional sense. My husband and I were involved in swinger activity when I said that, never did and would never do the open marriage thing because all that we did, we did together. For me to go solo (as in what I jokingly threatened) would be a violation, hence it being something to threaten with. I don't know what thought you're seeking. Are you for it or against it? Are you looking for details from others who've been in an open marriage? Without knowing any of that, I have no advice except don't do what YOU don't want. Don't let yourself get talked into something. Don't "try" something you're not ready for. To me, it sounds like your husband is going through the type of conflict that'll eventually lead to unhappiness. He'll either cheat (on the low end of possibilities) or be really unhappy and unresponsive to you which will make you unhappy (on the high end). You need to firmly figure out if you're open to other people. If not, let him know it's a no go and if he's willing to walk away, get to gettin. With his mentality, you'll always be sitting there with your heart in your throat wondering if he wants to be with you or not.