Indeed, one of the more common attacks against it is the idea that there is simply no such thing as "biblical marriage" as equivalent to one man, one woman given the polygamy of some of the patriarchs and kings of the Old Testament. Thus, the reasoning goes, if God does not condemn polygamy, how can monogamous, albeit homosexual, "marriages" be worthy of condemnation? They are, in this, partially correct. It is true that God nowhere explicitly (more on that in a moment) condemns polygamy anywhere in the Old Testament and it is true that some of the patriarchs and kings were polygamous and yet blessed by God. So how can this be if it is true that God's plan was always monogamy? But they conveniently choose to leave out the following facts:
Genesis 1 tells us that God, in making humanity "in his image" created one man and one woman in a relationship (marriage) designed for fruitfulness and mutual blessing. There are no indicators that any other kind of relationship was ever part of God's original design.
Genesis 2 speaks of God creating and then bringing the woman to the man as his perfectly suited companion. Again there is no indication that multiple women, or indeed, multiples or singles of anything or anyone other than a woman would be the ideally suited companion to complete the man.
In Genesis 4, we meet Cain, who is not only the first murderer, but also the one who sets up a civilization opposed to God. One of Cain's descendants (Lamech) not only doubles down on Cain's murdering, he is also the first polygamist. Say what you will, this is hardly a recommendation for the concept.
Or, if you want to get actually into the details, consider the four major figures who were polygamous in the Old Testament. All were blessed by God, but it must have been in spite of their polygamy, because their polygamous families are all presented in their respective narratives as a mess you wouldn't want any part of. Consider first Abraham: Abraham married Sarah, Hagar the Egyptian, and Keturah. He had Isaac through Sarah, Ishmael through Hagar, and six sons through Keturah. Hagar and Sarah were at war when they lived in the same household and Hagar was eventually "sent away" (i.e., divorced). Her son, along with the sons of Keturah, formed the Arab and Bedouin tribesmen that were at war with Israel (the sons of Abram's grandson Jacob) from 1500 BC to the present day. So that worked out well.
Now consider Jacob: He had two wives, Rachel and Leah, along with two concubines, Bilhah and Zilpah. There was unrelenting competition among the legit wives, who each gave their handmaiden to Jacob as an additional wife. The whole sordid story, including Leah "renting" Jacob from Rachel in exchange for some of Reuben's mandrake roots, the selling into slavery of Joseph, the firstborn of Rachel, and so on makes one wonder "How can God be using these people to redeem the world?" but it never makes you think, "If only I had some more wives, because this looks like a good plan that God blesses."
How about David? Well, one of his sons (Amnon) raped his half-sister Tamar, in recompense for which he was murdered by his half-brother Absalom. Absalom then, after a complicated series of events, led a rebellion against his father David and took the kingdom for a time. This rebellion was due, at least in part, to the fact that David was not going to give the kingdom to him, but to the son David had with Bathsheba, whom David had gained as a wife through seduction and murder. That son, Solomon, had his half-brother (Abijah) executed because Abijah was scheming for the throne as Solomon's older brother by a (more) legitimate wife. So again, this seems like a pattern worth replicating, no?
Solomon, the all time biblical polygamy champ, was "led astray" from the Lord by his many wives, who introduced explicit idolatry into Israel again. He is in fact the living embodiment of the reason for God's command in Deuteronomy 17:17 that the king "must not take many wives for himself, or his heart will be led astray. He must not accumulate large amounts of silver and gold." Moreover, because of Solomon's violations of these very commands, his foolish son Reheboam lost the northern half of the kingdom to a former general who set up idolatry, continuing the worship that had been imported along with Solomon's wives. The spread of idolatry, which grew to prominence in precisely this way was in fact the reason for the eventual exile from the land of both northern and southern kingdoms.
Moving to the New Testament, Jesus emphasized repeatedly that "At the beginning of creation, God 'made them male and female.' 'For this reason, a man will leave his father and mother and be united to his wife and the two will become one flesh.' So they are no longer two but one." (Mark 10:6-8). Note that Jesus goes back to Genesis 1 and 2, emphasizes the original pattern given by God as equivalent to God's plan for marriage. Also note the following: 1) male and female; 2) "wife," not "wives"; and 3) the repeated use of "two" as the number denoting a proper marriage. Jesus doesn't support the polygamous idea as anything other than a corruption of God's ideal.
Further, in the list of requirements for church leaders (elders and deacons) in the Pastoral Epistles, the Greek term mias gynaikos andra (literally, "one woman man") is used to indicate that the proper number of wives for a Christian leader is one.
Thus, there is simply no evidence for the claim that biblical marriage has a wider definition than that of the one-flesh union of one man and one woman. Not that I think this will convince anyone not already inclined to accept the Bible as authoritative and true. That is, I don't believe that anybody making this argument is doing so as anything other than as a way to tell Bible believing Christian to shut their collective pie holes already. But at least you can point them to what the Bible actually teaches on the subject rather than what they seem to think that it does.