In the years I have been interested in polygyny, the subject of what is morally allowed between a man and his wives, or even what is moral between wives, come up many times. Over the years I have read many posts and articles on the subject. It is apparent the possibility of a man having two wives in his bed stirs people up on both sides of the issue. I understand people having strong feeling on the matter, whatever they believe about it, but as Christians, the relevant issue should be what does the word of God say on the subject, NOT how we "feel" about it.
Now the law God gave us is recorded in the Bible back in Leviticus and other books contemporary to it, like Deuteronomy. In searching those books you will find that male homosexuality is clearly prohibited, along with beastiality for both men and women, and about every kind of incest one can think of including those involving polygynous families (father's wife, a woman and her daughter) adultery is also clearly prohibited.
I would say what is missing here, but that would imply YHWH was not thorough and left something out, so I'll just say what is NOT here is any limiting regulation on what a man may do with his wives, or what those wives may do with each other, in the context of their marriage.
People that would condemn certain activities often cite Romans 1:26 as a prohibition, but lets look at what that verse says, and keep in mind that they would have in no way been adding a law, as God's word forbids that explicitly! This means that whatever the author had in mind would not contradict, or add to, the rest of the scripture, but would rather have a foundation in it.
Romans chapter one, if you read the context leading up to the verse in question, is describing people who were unthankful, unrighteous, and involved in idolatry, then it begins in v26 with "For this reason God gave them up to vile passions. For even their women exchanged the natural use for what is against nature." When you look up those words natural and nature you find there are two Greek words. Natural is translated from the Greek word Phusikos (Strongs 5446) Strongs says this is taken from the other word rendered "Nature"(#5449 Phusis, see definition below) and that word is in turn taken from one that means to puff or blow (as in swell up) germinate or grow (sprout, produce) I think the idea here is pretty plain.
#5449 Phusis, from 5453 growth (by germination or expansion), i,e. (by impl)natural production (lineal descent); by extens. a genus or sort; fig. native disposition, constitution or usage:--- ([man-]) kind, nature ([-all])
Now while lesbianism, as it is called today is contrary to natural reproduction, I doubt seriously in the patriarchal society we are discussing, where the men were the providers, that this is what is being referenced here. There is no mention of these women leaving their husbands to be with women. In the next verse we have Paul saying likewise the men, leaving the natural use of the woman, burned in their lust for one another, men with men, committing what is shameful, and receiving in themselves the penalty of their error which was due.
The phrase I would like to call attention to here is "leaving the natural use of the woman" because I really believe that is the key to understanding what is being discussed here.
If you start with what the men do with each other, which God has clearly called abomination, and apply the "likewise" backward, to what the women were doing, you realize there is an un natural use, which is against "nature" (procreation) which God HAS already addressed in the old testament, and which could be participated in by the women and their husbands.
The total absence of biblical condemnation for what the world calls "lesbianism", should give one pause, and may be rooted in who God holds accountable for the activity of the women. Back in Genesis when Adam tried to blame his wife for his actions, God told the man he would now "rule over his wife." The places in scripture that allow a man to over rule, or make void, any vow that his wife may make, prove that he is the one responsible to God for her actions, as long as she is his wife.
Another scripture relevant to this is the parable of the ten virgins. The five who were wise went into the marriage chamber with the groom. Nothing is said about what transpired there, but one can only conclude that anything not prohibited in the law is allowed.
A final thing to consider, is that the word God uses to condemn the most vile of acts is abomination. There is a sobering verse in the proverbs that states "He who justifies the wicked, and he who condemns the just, both of these alike are an abomination to the Lord." Proverbs 17:15
As believers our words and actions need to support God as the ONLY moral authority, and affirm that His law is complete. Without this solidarity, how can we hope to sway any other to our belief?
If anyone has scripture that is relevant to this discussion, or challenges the position I have stated here, I hope they will bring them forward. The truth sets us free, but only when we are willing to let go of error. May God grant us the desire and the strength to do so.
Other subjects related to this are the purpose God had in creating woman, which does condemn women who think they don't need a husband. "The woman was made for the man"
Also one can ponder the implication of the verses that state that those married do not have authority over their bodies, but rather their partner does.
And finally one can try and discern the implication of those verses that relate to cleanliness following sexual relations, and see if they would have a limiting effect on a man with more then one wife.
Just as a side note. There is a verse in Ecclesiastes (1:9-10) that says there is nothing new under the sun, and that which we think is new has already been in ancient times before us. Deut 22:5 says a man is not to wear a woman's garment, and a woman is not to wear that which pertaineth to a man. Since sex has existed from the beginning, and mankind has perverted it for as long as history records, it is possible that what "pertains to a man", especially a warrior, is his manhood. A modern woman trying to be a man in dress, and attitude, often uses "strap on" devices and, if Ecclesiastes is correct, women may have done so in ancient times as well. Just something to think about.