Plato was against the theater for two reasons.
Reason number one: Plato believed, like other Pythagoreans, that there was a real world of mathematics that was very different from the world we live in. He thought that everything we saw in this world was a false and distorted view of the pure reality in the mathematical world. It was very hard, but through your thoughts and the application of pure mathematics, it was possible to get at least a better view of the true nature of reality. He very famously said, "If we are to discuss the cosmos, we must not look at the stars."
Today this sounds like the statement of a man who is insane. After all, science has learned so much about the cosmos by studying the stars. But in Plato's mind anything here in this world was at best a poor imitation of what truth there was to be found in pure thoughts and mathematics. How does that apply to theater? Quite directly. Theater to Plato was a group of men acting out a very poor imitation of our already very poor imitation world. A bad copy of a bad copy, in other words.
Therefore any play would only confuse people. It couldn't possibly teach you anything except false ideas about our already false world.
He also was upset about the theater and fiction in general because he felt people were very impressionable. If you told them stories about people doing bad things, they would be tempted to do the bad things. Since many of the Greek plays told stories about jealous, angry and unworthy gods, he feared that the plays would teach people to disrespect the gods. He even thought that if plays about suffering in the afterlife were watched by people, they would become so afraid of what might happen when they died that they would no longer be able to do a good job of living.
Today we are likely to say in response that plays and stories can also teach people good things and that perhaps they either balance out or even weigh in on the side of the good. Plato felt any good presented by a play wasn't real good because it wasn't actual goodness, it was only a bad copy of false goodness made from a bad copy of false goodness in the real world which was not the pure mathematical goodness of the higher realm.
Even worse is the idea that a playwright would be a good person to teach morality. Plato thought the teachers of morals should be people specially trained to instruct in this area. Obviously playwrights weren't so trained and therefore would do a bad job of it. But he was an absolutist, a utopian. He thought that he could create a perfect utopian society as long as it rigidly and absolutely controlled instruction and behavior in every single aspect of life. Today the teachings of Plato on government sound an awful lot like the teachings of any police state, whether Communist or Nazi. Many people have criticized Plato for his rigidity and trust in total government control.
Aristotle said that this was nonsense. Plays didn't make you have bad emotions, we were born with those. What plays did was help us learn how to handle those emotions and to see the bad possible consequences of letting them get out of control. He also said that watching a play about terrible events would lead to a catharsis which made us feel that now the bad thing was over, thus generating healthy attitudes. This would help us to be stronger and able to better deal with real life.
And by the way, Aristotle believed that the real world was real. He thought the best way to learn about things was to study them and even do experiments about them. One of his pupils was Alexander the Great. Once Alexander was a grown man and busy conquering the world, he took a lot of time to collect specimens of strange plants and animals that he could send back to Aristotle so that he could learn about these new creatures and the new ideas of other lands.
Plays have always been a problem, and so have been books. A few years ago in our local library here in Apple Valley there was an attempt to ban a book on anime and hentai. The book was in the adult section, not available to children, but it did show the sexual side of these Japanese arts. In the end it was decided to let the book stay because, after all, it was just telling facts about this Japanese art form, not advocating the sexual aspects.
We still hear of people trying to ban Huckleberry Finn. At first people wanted to ban it because it presented Black people as full human beings who were not inferior. Now people want to ban it because it keeps using an offensive term applied to Blacks, and people think it's racist.
There are always reasons people want to censor. It is very hard to make an effective balance. Books in Germany are not allowed to deny the Holocaust. Books in America can. America bans child pornography. America does not ban pornography in which beautiful women step on and crush small animals to death.
The Japanese have a version of hentai called guro which involves horrible torture mutilation. It is considered an acceptable artform in Japan and has roots in the 1930's. Most people are discomfited by it, even there, but the Japanese think that people who like that sort of thing will be able to get their satisfaction from these cartoons rather than from actually hurting real people or real animals. Are they right? No one really seems to know. You will get people who have studied the subject saying yes and people who have studied the subject saying no. Obviously we need a lot more study!
I conclude this argument by stating that Plato was being foolish, blinded by his utopian dreams of perfection in society, while Aristotle was being reasonable and thoughtful. Human nature is a real thing. Utopians always think they can change human nature if they can just have enough power. The problem is human nature doesn't change. If it did, we wouldn't be human anymore. We would've evolved into something else. We humans are storytellers. As far back as we can trace, we have always told stories to each other. This is natural and normal. It is one of the most important ways we learn. Plato just couldn't face these facts because he didn't like them very much and they made all his pretty dreams seem impossible. Aristotle faced facts whether he liked them or not.
My final personal comment. Someone once very famously said that all philosophy can be found in Plato, all the rest of the subject is merely commentary upon him. I'm willing to agree with that point as long as you are willing to add that the commentary is on how wrong Plato was about just about everything.
Oh, and let me add, if you're upset about the whole idea of guro, think about some of the really ugly American horror films made lately. Things like Saw and the Human Centipede. Yes, I know Centipede wasn't American film but it was very popular in America. We like guro too. We just don't call it that.