Okay, I’ll bite. I’ll play “some Catholics” advocate.
The reason why “some Catholics” see asexuality as a sin because some people take it too far.
A normal person that experiences asexuality, we wouldn’t call them asexual we would just call them celibate. Said person recognizes and accepts the gender that good God in heaven asigned to that said person at birth. Where people take their asexuality too far is when you start identifying as an alphabet person. You see your asexuality as an excuse to say that you have no gender or that your gender is fluid. Which is obviously wrong.
So my theory in the how these “some Catholics” are thinking is they love you so much that they’re concerned that you’re a sexuality might turn into a full-blown gender dysphoria. Some of the other posters here suggest that you should probably see a doctor just to be safe. If I were you I would probably see the doctor just to make sure you’re not experiencing any hormone imbalances. If the doctor(s) say that everything is completely fine and you’re not experiencing any hormone imbalance, then consider your asexuality a blessing in considered becoming a monastic. Not having to worry about getting married or finding a mate frees up a lot of your time. Time you could dedicate to Worship in our Lord and providing good works so you could bring Him good fruit.