haha, I like the solution, though I fear it's gonna be hard to enforce. "Sir, open up! It's the police. Are you letting your wives have sex with each other? Right now? Yes? Well... let me just make sure then."

I think it could also be accomplished by three-way marriages: everyone is an equal partner before the law and married to each other (each person having equal rights to benefits, pensions etc of all spouses). Although this would first require the marriage between people of the same sex to be recognized. You also wouldn't solve the problem of private power structures, but I'm not sure the law can anyway. Certainly not considering groups like the FLDS are based on the idea that female subjection to men is demanded by their God. And the fact that it's not just the men who feel that way.