It's a funny story, actually.
I'm aromantic, and ever since I was about ten years old, I found the thought of a relationship with anyone, male or female, to be pointless (not gross or repulsive, but I never saw the point). Of course, at such a young age, whenever I expressed such thoughts, people told me things like "You're only ten! You'll change your mind about this when you get older." So I went with it. After that point, I pretty much just assumed I'm straight, and that I needed to grow. I was wrong.
Whenever my fellow classmates in middle school would ask me "Do you like anyone?" or something along those lines, I would simply respond "I'm waiting until I get older to do any dating." A lot of people thought I was weird for that, but I wasn't really interested in anyone in a sexual or romantic way at the time (nor am I today), so I didn't make a big deal out of it. So basically, from ages 10-13, I pretty much started taking on the assumption that I'm straight and just need to grow.
It wasn't until I was about fourteen that I was started to come back to the thought that I might be asexual. I knew what the term meant for a long time. Ever since I was about twelve years old, I was already aware that the term existed, but never strongly identified with it. At fourteen, I'm hearing all my classmates talking about how they want to date or fuck someone, and I was so confused. However, at fifteen I especially started to change back to the thought that I was straight or maybe even bi. This wasn't accurate, though because I was simply experiencing aesthetic attraction. I could look at someone and think they looked pleasant, but I never actually thought to myself that I want to have sex with them or even date them.
The conclusion that I'm asexual came when I was sixteen. We all know how popular porn is. So many people, especially classmates, talked about how porn is great, how they can't live without it, and how I'm "missing out" by not watching it. So I was curious, and I decided to simply look up some pornographic GIFs (although not actual videos). And...
I WAS EXTREMELY REPULSED AND NEVER WANTED TO SEE THAT AGAIN.
Pretty much, that moment gave me an incredibly repulsive view of sex, and from that moment, I concluded I was asexual. Both gay and straight pornography grossed the hell out of me, and I felt like an extreme weirdo for not being able to understand one bit how people can watch that shit without wanting to destroy their eyes, let alone get addicted to it.
I took interest in asexual communities after that point, and I realised how much I have in common with common ace "ways of thinking." I found sex repulsive, I never understood why people made such a big deal out of it, I was content with the idea of being a virgin all my life, and I would pretty much rather do anything else over sex, and I'm okay with that.