For polygamy to be legal it must be consensual, entered by adults, and absolutely equal.
This is against the beliefs of muslims, who claim that god has made men superior, and women inferior, bound to obey and submit to their men who have the right to force them into polygyny and beat them if they object.
So, I believe that in order to marry a second spouse, muslim men (and all others, with adjustments) should be obliged to take the following oath:
I marry you, NN, as my second (third, fourth et.c.) wife with the full consent of my prior wife/wives. By doing so I proclaim, before Allah, you and all of mankind, that I wholeheartedly and willingly accept your absolute and equal right to enter marriage to more men, or women if you prefer. I pledge myself to encourage and support you in polyandry/polygamy and I declare before Allah, you and all mankind that I do this with all my heart, all my soul and all my mind, holding your rights in every way to be the same as mine. My wives’ equal rights in every aspect of life, I do hereby declare to be a law of Allah, a law of nature and a law of society. I will accept any spouse you, or any of my wives, may bring into our polygamous family as your legal spouse and my co-spouse. I hereby declare that if I ever retract, verbally or mentally or by any kind of act, any part of this oath, if I have, or ever have, any proviso, restriction or reservation in giving this oath and keeping this oath, I commit my soul to burn in hell eternally. Ameen.
The day all muslim men agree to take this oath as part of their nikah, is the day I will respect muslim polygamy.