People frequently claim that the slippery slope from gay marriage will be polygamy (forgetting that polygamy has been around for thousands of years). This is why I predict that we won't see plural marriages on every street even if polygamy does get legalized: it's expensive.
You will either need to be rich or else most of the spouses in the marriage will need to work in order to financially support a plural marriage. And, let's face it, a good chunk of those interested in plural marriages come from culturally- and religiously-conservative traditions that do not encourage moms to work outside of the home.