The point being, I spend most of my disposable income on media of various sorts, but that doesn't mean I can afford everything I want
To play Devil's Advocate: Why do you think you deserve everything you want?
and if I can have it, why not? No one would be getting my money if I didn't 'steal' it, so the only person losing out would be me.
Except that you already stated that you "spend most of [your] disposable income on media of various sorts." Perhaps at some point you were presented with two options, and could only afford one of them. Further, perhaps one was easy to pirate, and the other was not. This is not unexpected if you're comparing a single-player vs. multi-player game, for instance (or a console vs. a PC game). Now, without the ability to pirate one of them, perhaps you'd get the single-player (or PC) game because you like it more. But when you add in the ability to pirate, you might instead buy the multi-player (or console) game, and pirate the single-player (PC) game.
Or we could compare types of media. Maybe you're more scared of the RIAA than of various game companies, so you pirate all your games, and buy all your music, even though you'd buy a sampling of each were piracy not an option.
Thus, even though having the ability to pirate media doesn't cause you to spend less money, it can affect where you spend your money. It adds artificial pressures to the market, and skews sales figures, based on what is easiest or least risky to pirate.