Theft: the act of taking something from someone unlawfully.
There are a lot of things that aren't fun, and I certainly agree that people pirating your software can be a problem, but there is a distinct difference between theft and illicit downloading.
Yeah, the analogy with theft, particularly physical theft, is unconvincing. Counterfeiting is nearer the mark, but there are obvious differences there as well.
My big complaint with these analogies, though, isn't with their accuracy or lack therof. It's that they shed absolutely no light on whether copyright laws are good, bad or indifferent. They imply a moral imperative for copyright laws. This is, to put it mildly, controversial.
The theft analogy has no bearing on the question of whether the bad effects of copyright laws outweigh the good. I'd rather see the copyright lobby concentrate on that question (other than by quoting implausible estimates of the cost of "piracy"). That, at least, is a defensible position that can be debated constructively.
There are a lot of things that aren't fun, and I certainly agree that people pirating your software can be a problem, but there is a distinct difference between theft and illicit downloading.