Unless you BOTH sign up for this agreement? It is not an "agreement" -- it is an order.
He is being fresh. In a marriage partnership you don't just "announce" a One Penis Policy. Is this dictatorship or a partnership? WTF? You are not a child. You are grown up person who can choose or not choose to what contract/agreements you sign up for with him. Because you draw them up TOGETHER.
Now if you BOTH come to agree on that, it is another thing entirely.
But I'd be resentful and wonder what ELSE Mr Dictator will spring next? You may never eat brownies? You may only wear pink shoes?
So I don't think your feeling all ---
...is unrealistic here. Total dbl standard. If you open, you are open on both sides, and you CANNOT dictate the gender of the person if you are wired Bi. What if you told him ok, you can only have women but then he can only have men then so it is fair that is is only of the same gender?
You guys need to talk this out and come to a more fair "Open Relationship Model" agreements here. For how to be in right relationship with each other while dating other people.