I feel your pain, and I would have at least one sleepless night in that situation.
I think you're right to be clear that you are too uncomfortable to hang out with L until she cools the drama and behaves as if she respects your relationship with J.
But, as your husband said, that's really J's issue to deal with. Is he dealing with it appropriately? How did he respond when L pushed him to hit on other girls?
(I would be majorly pissed off in your place. Why is it that women are so judgmental of each other? Why do they feel they can manipulate other women's relationships?)
And remember, the best revenge is living well. Keep on going about your business, and L will eventually get the idea that your relationship with J is awesome and healthy.
Maybe at some point, you and your husband can socialize with J and L, and you can talk to her openly about poly. (Probably not worth trying, though, as L sounds like kind of a narrow-minded tool).
Single, straight, female, solo, non-monogamous.