Nope, I think that's exactly what I'd say to them. Family members who reject, make their own decisions as to whether they will ever shape up their attitudes. Could take a month, a year, or a lifetime. Depends on how stubborn they are. The best you can do is always be polite but firm, and be ready and willing to answer any questions that don't snoop inappropriately into your business.
The world is unfortunately not ready to accept polyamory. You'll probably have to make some difficult decisions about how to respond to, "We don't want our children exposed to it," etc.
Love means never having to say, "Put down that meat cleaver!"