If I sound like I'm saying something along the lines of "if you say anything to the contrary, you are simply trying to exuse away your own thefts," it's because within this particular framework, that's a valid response to the question as posed. In the arena of filesharing, "can we refute stealing is stealing?" In short, no. But here's what you can do: you can re-frame the debate.

Every argument that the side against copyright has seems to be perpetually bogged down in definitions and assumptions and challenging paradigms

There's a reason for this. Me? It's in my marked interest, as it is for anybody who works within and stands to benefit from the current system, to keep the debate firmly on grounds of morality. Because, barring a change to the system, it's not only an extremely easy argument to make, but it represents the last available appeal to people who would otherwise bypass that system entirely at the expense of a few distant actors. You can get this by means that lie outside of my preferred marketplace, but if you do, you are effecting my bottom line. That's an easy and potentially powerful argument.

You're going to have a hard time refuting it, too. On the other hand, if you as a file-sharer take up the argument on practical grounds, your job becomes easier: "whether or not my actions are moral is moot; available technology allows me to assign lower monetary value to the stuff I want. The onus doesn't fall on me to ignore available tools, it falls on the market to correct for the presence of those tools."

Then again, I can counter (and already have) with the argument others have taken up in other IP arenas. Sure, you can leverage available tools to take my IP, and there's nothing I can do about it. But if there's no money in it for me, I'll stop producing. That's a market correction.

Anyhow, the question about "how to counter the morality argument" seems a bit off the mark, as you're looking to fight a very uneven fight. Which is why you as a consumer are better off reverting to practical arguments, as others have done. But then I'm not sure that road takes you anywhere you necessarily want to go, either.