You have the right to think it is a piece to the puzzle. But, when it comes to testosterone levels, it is likely a corner piece that is not very important in solving the entire puzzle, when there are countless studies proving there are other things affecting human and other animals' test levels.
Watching porn can play a PSYCHOLOGICAL role in affecting ones sex life. But, not likely to cause much hormonal damage from I have seen or experienced. I have been the typical teenager, watching porn daily throughout my middle school years, and have never had an issue in testosterone levels. Finding it harder to get aroused around the typical female because I would be comparing them to porn stars? Damn straight, but as you get older you typically* grow out of that and are fine.
*Typically meaning, those that are not legitimately addicted. That would cause a whole different argument/conversation about addiction and how addictive habits affect hormones alone.
I am not saying you are wrong, or that you can't have your own opinions or hypotheses.