The original owner is potentially deprived of compensation for the service of providing the copy to you.
Of course notice how that is still not theft, and how I mentioned potentially. But the original owner is potentially deprived of that.
Of course then we need to get into whether the copy would have been made or the content would have been outright ignored without piracy... and whether the act of pirating the content will push the pirate to purchase content (s)he would otherwise have not purchased. Etc. etc. etc.
So you're not entirely correct... but anti-piracy groups are much much moreso not correct. Just google "Monty Python 23000%" or "comic 4chan watches sales soar" for examples of how piracy leads to sales which otherwise would have not occurred... and "Ubisoft Piracy and the death of reason" for an example of how a lack of piracy and methods used to control piracy HURT revenue.
Hey, eventually one day people may realize... control does not scale with revenue... and you often need to sacrifice one for the other... and to stop blaming the lack of the latter on the perceived lack of the former -- rather the superabundance of the former.