That polygamy should be legalised

One of the characteristics of our society is that it is democratic. Like Aristotle said, the underlying principle of democracy is freedom. Freedom is the aim of democracy.

Polygamy is a personal choice. Polygamous people choose to marry⁽¹⁾ more than one person.  It's significant to point out that all parties agree to do so. By criminalizing polygamy we are limiting the free will of the citizens. People must have the opportunity to choose between monogamy and polygamy freely. 

As the aim of democracy is freedom, democratic governments must ensure that their citizens have free will. One of the things not allowing this is the criminalization of polygamy. Polygamy must be legalized in order to enhance people's freedom.

I understand know that bad is a subjective term but what I mean by bad is that It isn't negative to the ones being polygamous nor to the others.

There is not negative effect caused by being polygamous. One of the justifications of limiting free will is self harm. However, polygamy doesn't produce any negative effects on the ones who practice it. 

Another justification of limiting free will is damaging the others. Nevertheless, polygamy doesn't  causes any harm to the ones around polygamous people. 

There are lot's of polygamous people living worldwide⁽²⁾. The problem is that, because people can marry with only one person, the other parties of the polygamous group can have all the legal benefits and protections of marriage Legalizing polygamy will permit this people to be protected by the law.

Con's argument here was that democracy is not "about granting every citizen everything they want because of personal freedoms". He also pointed out that "There has to be a line in the sand somewhere" in order to know what should be allowed and what shouldn't. 

I agree with this. There must be a line separating freedom from debauchery. This line is quite clear. The limit of our rights and freedoms should be the rights and freedoms of the others. To know if something will generate freedom or debauchery you have to ask yourselves this: Does this activity harm others? Does this activity limit or take away the freedom of other individuals?

Let's look at polygamy with this perspective. Does polygamy harm others? No, polygamy does not harm the individuals who don't practice it. Does polygamy limit or take away the freedom of other individuals? Again no. As you can see, polygamy is clearly on the freedom side of that line in the sand. Then polygamy should be legalized, and citizens should be allowed to chose if they want to be polygamous or not.

I can't give real numbers concerning this. Anyway, this does not disprove my point. Some of the rights we now take like granted, had a great opposition. Years ago, most of the people thought that Afro Americans were inferior, that women shouldn't have the right to vote, that communists were pigs, and so on. Does this mean that all does claims were right?  I don't think so...

Con's contention here was that polygamy can be harmful for the ones who practice it.

It may be. However this does not mean polygamy should be illegal.

Individuals who choose polygamy know what the consequences could be, but they choose to be polygamous anyway.  This is the same as with alcohol, tobacco, driving, air travel, junk food, and other stuff. All of this thing can cause harm to the ones who consume or practice them. Nevertheless, they are not illegal. Why? Because people know what could happen and they accept the risk. The same applies to polygamy. 

There are lots of activities that can produce even more serious harm that polygamy but are legal. Things that can kill you and the others around are legal, and we take that as normal. Driving, drinking, smoking, flying, eating junk food, consuming prescription drugs, having an operation, playing violent sports, and so on and so on. Why must polygamy be illegal then? If polygamy can only cause minor harms only for those who practice it, why should it be illegal? 

Even if it continues to be illegal, there will be polygamous people.   

This creates a new problem. If polygamy is a crime, It can't be regularized. This means, all those wives and husbands are not being defended by the law. If polygamy is not regulated, then all the polygamous people aren't having their rights ensured. 

But if we legalize polygamy, we can regularize it. This means we can ensure their right to all the polygamous people. By doing this, the government is closer to the possibility of protecting everyone. On the other hand, if polygamy keeps being illegal, government won't be able to protect all of its citizens.