She asked him about the alleged link between forced monogamy and democracy.
That's not a link I'd want to make too strongly [good - he shouldn't], but it has been argued by historians that monogamy precedes, and then seems to go along with, the emergence of democratic ideals. In the Western tradition, the earliest we can trace laws about monogamy is actually to Athens when the first notions of democracy began to be instituted. The argument is that it's meant to create equality among citizens so that, essentially, there'll be wives available to all Athenian men, rather than having all the rich men take many wives. Although, men were still allowed to have slave concubines just so as long as they were non-Athenian women.
He is referring to cultures and times in which women were essentially property. Monogamy kept one man from getting more “property” than another. But with full citizenship for women and gender equality, the dynamics are different.
You can think of it as a first kind of effort to level the playing field. By saying that both the king and the peasant can only have one wife each, it's the first step toward saying that all men were created equal.
But it was literally about men, not women.
The core of the argument is that polygyny -- when men marry multiple wives -- takes up all the women and creates an underclass of men that have no access to partners, and those guys cause trouble. They commit crimes and engage in substance abuse.
It’s a bad argument. See below.
Also, if you have one male with lots of wives, there are all sorts of stepmothers and unrelated adults in the same household as children, and that increases the likelihood of violence.
If everyone was kept isolated from everyone else, then sure, there would be less violence. Violent people are violent wherever they are. The important thing is to avoid violent people, and prosecute them when they assault others. It is ideal for a man who commits domestic violence to have no spouses (or children) with him, not one.
This guy sounds like those people who marry abusers thinking that marrying an abuser and loving them the right way will get them to stop being abusers. Marrying an abusive young man will not stop him from being abusive. It can make it worse.
Back to the “problem” of unmarried men…