What never computes about the OPP policy, for me, is that it is basically saying that straight women shouldn't be poly. And I know that my ability to love more than one has nothing to do with what kind of genitalia I choose to engage with.
I think it's also dismissive of female/female relationships in the first place, that it implies that they "don't count" as real relationships.
What always amuses me is that the wives/girlfriends of men who have a OPP could just as easily leave them for a woman, and in some states, mine included, they could even get married.
The only possible legitimacy I can see behind a OPP is the fear of pregnancy. It isn't unreasonable for your husband to not want to help raise another man's child. You two should definitely discuss what would happen if an accidental pregnancy resulted from another relationship. Outside of that issue, I think he needs to get over this.