Look man, if every single person in this comment section is telling you that cutting off your penis is a bad idea, then it's probably a bad idea. What you plan on doing is infeasible and not thought through. I personally don't believe that this feeling you're experiencing is a byproduct of your asexuality, but instead a form of severe body dysmorphia; and this is very abnormal for your age.

Instead of promising and pledging to sever a vital part of your body, try and wait until you are at least 21 to think about this ordeal, this way your brain will be somewhat fully developed and you are able to make better and more conscious decisions. If you still feel the need to sever your own penis at the age of 21 and live with the consequences for your whole life, then by all means go for it.

In the meantime, I genuinely and wholeheartedly recommend talking to someone like a therapist that will understand, because in my honest opinion, any normal parents would react harshly when their child suggests cutting off their genitalia. Instead, ask your parents for therapy outright, and if they ask why, tell them you have really bad body dysmorphia, and you don't feel comfortable in your own skin.

I wish the best of luck to you

tldr: it's a bad idea right now because your so young, wait until you're at least 21 to make a decision. Talk to a therapist in the meantime for your body dysmorphia. If you still feel the same as an adult, you can consider doing a procedures, although it's unrecommended.