I have to admit that when husband and I first started out on the poly path (and I am deliberately frasing it like that: it felt like something we were doing 'together') I have probably said this, to friends etc: it enhances our marriage.
I would no longer say that now, because the 4 years of poly have made me realize that this is so much more about me, than it is about us.
(I actually started a thread about that here
I would now say that it enhances ME, to have more than one loving relationship. What I do say sometimes to people who are new to the concept of poly, when I tell them I have a BF, 'oh but please know that Ren and I are also still very happy together', because I've noticed that people tend to think that we have other loves because we no longer love each other.
I agree that the level of discomfort your BF feels when you bring this up, is interesting. Because I am married with a (until recently single) BF, I tried to put myself in your husbands shoes and thought what I would say if my BF had asked me this question.
Could it be he feels guilty that you are his only partner and he is happily married? Does it make him uncomfortable, that he feels he needs to 'everything' for you while he has several partners? This could be major projection on my part - but it's what my possible reaction might have been.