The article includes some of the polygamists' reasoning:
...Mark Henkel, who, as founder of the Christian evangelical polygamy organization TruthBearer.org, is at the forefront of the movement. His argument: if Heather can have two mommies, she should also be able to have two mommies and a daddy. Henkel and Hammon have been joined by other activist groups like Principle Voices, a Utah-based group run by wives from polygamous marriages. Activists point to Canada, where, in January, a report commissioned by the Justice Department recommended decriminalizing polygamy.
It seems as if the slippery slope argument from gay marriage to polygamy may be valid after all.