Tell that person that whilst it may not work for them-it works for other people and you weren't asking for their permission.
There are plenty of examples of poly working in the world-however you'd like to define "working" (argument abounds on that topic).
lovingmore.com also has some historical info.
Deborah Anapol has several books out-but I suggest just going to the bookstore-finding any one of them and reading the back cover-about her and her experience. She's got years of being poly under her belt. Self-defined as "successfully poly".
"Love As Thou Wilt"