I woul first of all try to make clear to him, that he overuses the Golden Rule by judging your behaviour as wrong, just because he himself would feel treated unfairly, would feel uncomfortable and wouldn't want to be in such a situation as a poly relationship. ("One should treat others as one would like others to treat oneself.")
And stress that everyone involved agreed to this and that he can't dictate what others believe to be appropriate and acceptable based on his moral code. That his moral code may be backed up by the common one from society, but that that one is basically an agreement as well and that the agreement in your situation has been made, but on different terms.
Before he realizes that there are other points of view, different from his own, as valid as he believes his own to be, he won't be able to follow any of your arguments at all.
Facts: 30, female, bi, v-type relationship with Sward (husband, straight, mono) and Lin (boyfriend, straight, mono), poly-fi and co-primary.