With polygyny, you have more children than one man can support. With two husbands and one wife, you have no more children than one woman can produce, but you have the earning and nurturing power of three adults to raise them.
Three adults can trade off sitting up with a sick child or a sleepless baby. They can take turns babysitting and let the other two go out for a night of adult entertainment.
From a kid's point of view, there's always a dad to help you with your math or show you how to throw a football. And your mom is probably more cheerful and relaxed than other moms.
This could be a solution to middle class financial stress. For those of us making under $100 grand, it should not only be legal, but encouraged.