I would say most of my argument comes from implicit sources within Scripture. I do think it is notable that in the beginning when God created the world and all was "good," He created one woman and one man to begin to populate the earth. In Song of Solomon, the dialogue is between one woman and one man. In a general sense (save for the example you gave), the context in which marriage is spoken of across Scripture carries with it an implication that God's design for marriage is that it be between one woman and one man in a lifelong, monogamous union.
No argument that marriage is between one man and one woman, so arguments that it is will only be agreeed to. It is that last addition about monogamy that you add in, that is in dispute.
Let me challenge your implicit sources within scripture with my own example: What is the Bible word for polygamy? It is "marriage". The Bible does not even differentiate it. It never treats it as something different or strange. We only see polygamy as different from monogamy because of our Western Civilization heritage. The Bible does not reflect this division. Polygamous marriages were just as valid as monogamous marriages. The nation of Isreal was built on a polygamous marriage. Many of the ancestors of Jesus were polygamous. As far as the Bible is concerned it is just marriage.
To give one explicit defense, however, I would point to Titus 1:6, where one of the qualifications laid out for those who would be appointed as elders/overseers is that they be a "husband of one wife." Does that make polygamy a sin? In the interest of intellectual honesty, I'm not sure that it does, however if it is on the list of ideal characteristics for those leading the Church, I think that only supports the idea that God's design for marriage was monogamy.
There are a lot of problems and ambiguities with this argument.
First, notice that this word translated "one" here is the same word that is translated as "first" in Acts 20:7.
Some here argue that polygamy was not even a thing in the 1st century. If this is true, then this verse can not be a reference to polygamy.
There are at least 4 different possible meanings to this verse:
1. Must have a wife (be married)
2. Must be married to first wife (wife of youth)
3. Must have had no more than one wife total in history (never divorced)
4. Must have one and only one wife (must not be polygamist)
For those who argue that polygamy was not a thing in the 1st century then this verse must have nothing to do with polygamy and must mean one of 1-3.
If polygamy was a thing in the first century then it is still possible that meanings 1-3 are what Paul was getting at. We simply have to read the total context and decide for ourselves to resolve the ambiguity.
Here are two reasons why this verse is probably not the anti-polygamy verse some wish it to be:
1. It is too weak. If polygamy is wrong and a sin why not just say so clearly here or somewhere else?
2. A list of qualifications for leader is the wrong place for breakthrough teaching on morals. Surely it would have been discussed in more depth someplace else in scripture? It isn't.
Worst case scenario for the polygamy supporter is like you say: it is not a sin. It is just a polygamist can not be a church leader.
It is interesting to weigh this with the idea of levirate marriage that you brought up earlier. This is purely speculation on my part, but in the Old Testament, levirate marriage is a command given for the widow's sake, but in the New Testament there seems to be a shift in how widows are to be cared for, that the responsibility belongs to the whole covenant community meeting their needs rather than a close family member. It makes me wonder if part of this shift is due to shift in cultural practice.
Or that the Gentile believers were not familiar with the levirate practice, or obligated like a Jew was? I do not know.
Perhaps a more interesting question, is would it be wrong for a Christian today to do it?
By the way, this has been a really interesting topic to think through more in depth. I appreciate the fruitful discussion thus far.
Thank you. I have enjoyed the discussion as well. I hope they do not delete it!
God bless you.