A few arguments I tend to see:
#1
-Piracy is wrong because it robs money from the developer, and doesn't add to sales numbers
-neither does buying used. Buying used and pirating are the same thing, as far as the company is concerned and the money they get. This is a point to be considered with the other arguments as well.
#2
-Piracy is wrong because copyright laws says only the copyright holder has the right to decide how the game is played, what medium, form, etc. It robs the creator of having control of how their product is enjoyed, even if they have no current way of sharing it.
-Yes, that makes it legally wrong. But this point hinges on whether you feel copyright law is fair and just, and most people would argue on that point. That's a whole other rabbit whole to go down, arguing the validity of copyright laws and the nuanced merit of owner vs general public. This applies to emulation of consoles and games as well, as much as some people wish/think it didn't.
#3
-Piracy is wrong because it is motivation to not buy new games. Why bother when you can get it for free?
-No argument here. There will be many people who will just pirate rather than play. But on the flip side, there will be people who will buy the game new because they got a chance to play it first, and decided they loved it. It allows for low-risk playing of games before purchase, so the gamer can make their money count and buy only high-quality games. This point is both a positive and a negative, and people love interpreting the numbers various ways. I wouldn't call this a strong argument, but maybe the strongest of the bunch.
#4
-Piracy is wrong because I was told it was wrong/feel like it's wrong.
-That's a you problem. Find a moral or legal argument rather than basing your argument on feelings or reliance on authority
I can't think of more at the moment, but I'll add more if I think of them.
Piracy isn't the same as outright stealing from someone else. It's more akin to one person buying a book, then copying each page of the book for all of that person's friends. They aren't buying the book, but no books were stolen from the seller. You can't say a sale was stolen, because that assumes far too many things to be a solid argument. I can see someone thinking it's wrong for some of the reasons listed, but I can see why many don't think there's a problem at all.
Most people don't understand much about copyright, or why it might be the way it is. I come across this a lot with photography. People just don't get why a photographer owns the rights to their photo. Take a photo in public? It's the photographers, no matter who is in the actual photo. Hire a model for pictures? It's the photographer's, no matter who is in the actual photo. Hire a photographer for a wedding or event? Yup, it's the photographer's, but is dependent on the signed contract. The client is entitled to a certain amount of agreed on photos, and the photographer has control over the method of delivery (no you can't have every single RAW picture and print them out at Walgreens or post them to your facebook). It's different for video games, but the basics are hard to understood for many. I still think copyright is broken, by the by.