It is very difficult to say whether you are asexual or not. Apparently, you are still a little girl and have never thought about it. But since you masturbate and imagine beautiful guys, you may be just afraid of sex. This is also a phobia, but it does not apply to asexuality. Asexuality is when you don't have a sexual attraction to people. Still, since you experience such feelings when masturbating, I think everything is fine with you, you're just not ready for sex yet, and you need to wait. I also used to watch only porn, and at the same time, I watched it only on this site --
https://www.vrpornhot.com/, because there is a lot of VR porn here. I imagined some kind of having sex with a girl and was preparing for the future.