How strong are you? You have two jobs here:
1) Sustain your relationship with your husband
2) Build a new relationship with J
It's not your job to make them get along, and they never have to, but if your husband doesn't want J as a roommate, you'll have to come to some other arrangement.
I'm not sure whether or not your husband actually agrees to polyamory, or not. The fact that he wants you to 'give it up' if you get pregnant pretty clearly implies it. You say you two know what the problem is, and what his problem is, but that's not immediately clear. If you get pregnant with your husband, will J still be around to help? Will he want to be involved? Will your husband let him?