Why is it morally wrong for me to have 2-3 wives, perhaps 2 wives 1 long term GF?

It is wrong, because the bonding of two people (man and a woman) is shown to be the perfect mix to raise children. 

You have one father figure, and one mother figure. The one father figure is supposed to raise based on his beliefs and principles. As soon as you have more than one father, then there different ways of thinking that is taught to the child. Then the child is confused. Then as soon as you have more than one mother, then the same thing applies. Mothers and fathers teach different things. So if you had more than one of both, then the child would be confused on who to trust, who to believe, and who to base there life principles on.

How is this shown exactly? What evidence do you have that proves this claim?

Or you raise your children to believe that there is more than one perspective in the world and it's a persons responsibility to question and consider the beliefs and principles presented to them and come to their own conclusions based on their observations, experiences and reasonings. In fact I'd say that giving a child only one voice they trust and believe is a bad idea. I learned from my parents who had different views in many things and my aunts and uncle and grandparents all of whom had many different perspectives on many different topics. This worked because they never taught me that their beliefs and principles were the only valid beliefs and principles, but instead taught me to consider what people tell me and make my determinations based on that consideration and personal experience.

You can teach this with two parents as well. Teaching your kid that our world views might be different than other family's. It is important for a child to have a figure that they can trust with different roles for different problems. Mixing in more than two would confuse the child. You can't make beliefs without a base belief. What are they going to base there beliefs off of, if they don't even know whos view to base there own beliefs on. Children aren't born with the knowledge of how the world works. You have to teach it to them. 

.. I based my beliefs off of my views, it's true that other people informed those beliefs, but it wasn't an individual it was a group of people who I spoke with, questioned and trusted for a long time. The last sentence is also immaterial to this discussion, as more than one person can do that. As I said, I learned how the world works from a very present and involved extended family as well as my parents. None of this seems to suggest only one parent is necessary or preferable unless you want to instil particular beliefs in a child.

The impact of the decline in marriage/the family has already been felt in poor communities, in America at least, and it's been a total disaster. That happened for the same reason that people are pushing polygamy today, high IQ liberals who were perfectly capable of having safe sex with a few partners before settling down didn't realize that the whole world wasn't like them. A lot of people in less educated communities got married only because of the taboo against premarital sex and the extremely strong taboo against illegitimate births. Marriage has a lot of positive follow on effects that stabilize communities and families that got totally wiped out. In some places births to married parents have all but vanished, which causes a lot of harm to the children. Stability is one of the most important things for childrens wellbeing

It's a bit silly to go blaming this on high IQ liberals, dude. It was always coming. Birth control and the internet and cars and planes and education and greater freedom in general, the whole lot, it all plays into it. Wasn't some dangerous idea, it was a dam burst. And it won't be dammed again. And we'll be fine. What's to be so unhappy about? Church raping kids wasn't much good for children's wellbeing either. That's the Holy All of it. Welcome to humanity. 

Educated and wealthy families almost universally raise their kids in stable two parent households. It’s possible that they’re unenlightened and backwards but I think it’s more likely that it’s just a social model that works

Any woman I ever fucked was clingy as shit, and that's not a small sample size.

Many sources, I learned from both my parents, my grandparents, friends of the family my aunts and uncles. I learned that people have different perspectives, beliefs and ideas. My father for example was an atheist, while my mother was a deist. My grandmother on my mothers side was a spiritualist while my grandmother from my father's was a christian (as were a couple of my aunts). All of these people were present for my childhood (with work I spent much of my time with my grandparents and aunts for example).

You have just proved my point. You grew up with two parents, and that gave you a base for your ideas and beliefs without confusing you. Sure you learned stuff from your grandparents, and aunts, but your base for what you believed came from your parents, because I assume you trusted them the most. What happens when a child is confused on who to trust?

Not at all. I spent a lot of my formative years questioning the beliefs of my parents and my grandparents and my aunts and uncles. I didn't trust my mum and dad more than my grandparents, I didn't trust them more than my aunts. I questioned all their teachings, I was fortunate that my family taught me to question and were clear in explaining that the beliefs they held were to be questioned. The flaw in your argument is that the child would be confused on who to trust. My point is that just because their are multiple points of view being presented to a child doesn't have to be confusing. 

Ya know, I buy polygamy or swinging for ugly people actually. Nothing to lose, everything to gain kind of idea. But that's a bit mean.