And you know this how? Conducted a poll? Did a study? Tried to start a polygamic family? Just "feel it in your bones"?
Actually, this article shows that you're very wrong. There is an organized movement for this, it seems to be growing, and it seems to have its roots in "christian" circles. If you think the movement doesn't include females, you know very little about human behaviour.
therefore the overall impact on society will be very small and the social costs trivial.
Ah. Let's play Heinlein for a bit, shall we?
First of all, these groups will be able to divert public resources to their interest. "Moving-to-the-mainstream" outlier groups like this always have. So if you count on limits built into the social security system and tax code to help, you're out of luck - they will be changed.
Second, they will be able to circumvent such limits fairly easily. You don't have to look further than the next Jerry Springer show to get to see specialists on that, probably.
Third, the polygamy angle will give society a number af strange new family structures, all claiming legitimacy, and all sucking the state/federal teat. You will (and that's a certainty) for example see "families" of, say, a group of gays plus a group of lesbians - rather like an old-fashioned collective but enjoying all the public support of a traditional family. They will have social benefits like a family, they will have tax deductions like a family. If they adopt enough kids (which they will be allowed to), you may even get to pay them taxes.
Bottom line: this whole thing will be costly, very costly. You haven't even begun to see the ramifications and you'll sure as h*ll not see them by staring myopically at two lines in the tax code or something ;).