No, but it could make you somewhat disingenuous. The argument you used is mostly irrelevant - piracy isn't necessarily stealing but that doesn't make piracy ethical; much in the same way that arson isn't murder, but that doesn't make arson ethical either. Piracy is piracy, and you should be capable of developing an ethical stance on it by its own nature.
Here's an example: If someone writes an original piece of fiction, do you believe he has the right to charge people for his story, or do you believe everyone is entitled to it? Is it within your right to demand that you have the opportunity to read through the story (or part thereof) before you choose to pay for it - even if he doesn't want to agree to these terms and would simply prefer you -not- read his story if you weren't going to pay for it?
If you believe you have this right, then you would think piracy ethical. Should you? That's up to you to decide.