I guess you might be able to understand his jealousy by thinking of it this way. Imagine that your emotional needs are equal in size. You meet your needs by giving and recieving affection with multiple partners. He invests all of those needs in you.
So, even if his needs are being met, he doesn't like that you give and receive affection in relationships that aren't with him. It makes him feel jilted (even if you aren't neglecting him), and possibly he feels like he is giving you more affection than you are returning (again, regardless of if it is true or not).
The fact that this is irrelevant to whether or not his needs are actually being met is important. It means that he can't be happy in a polygamous relationship, even if you would be giving him the same treatment in a monogamous one.
And that doesn't mean someone who is monogamous can't be happy with someone who is polygamous, only that this specific person can't.
Which isn't rational per se, but since we are talking purely emotions here, it doesn't need to be.