No no no no NO. It is NOT theft. Whether or not it's MORALLY right is another issue, but it is definitely not theft.
If you want to steal something, it has to be a real object, and you have to be depriving the previous owner of its use when you steal it. Neither of these things apply to downloaded music.
The thing is, to some people, music isn't a right, but a privilege. That's a terrible way to look at music that I definitely do not approve of, but there are people in the industry who think like that. And if those people don't want us to listen to their music without paying for it, that's fine. It's their product and ultimately their decision to decide whether or not I have 'stolen' music. The law might not agree with them, but until you're being slapped with a lawsuit then law never comes into these debates, they're entirely about who thinks they have the moral higher ground.
If you took a naked picture of your girlfriend and charged money to see it, that'd be fine. If I stole a look anyway and you called me out on it, that'd be fine too. If you told me that I had no right to take a peek without handing you £5, I would agree with you. Whether the picture is worth £5 to look at or not doesn't come into it. It wasn't mine to look at. Even though there's nothing material that I am taking away from you.
That's all digital music is. It's a 'look'.
This is not a debate about law and it is not a debate about copyright, despite words like 'theft' and 'stealing' being thrown around.
There are two individuals involved in this.
The artist and the consumer. They alone choose whether or not they are stealing or being stolen from and it is nobody elses place to say.
Respect the artist. Respect the consumer. And don't bring in some bullshit about copyright to defend a moral stance.