I mistook your gender as well, initially. In my defence, I had no idea what "Alicorn" meant, except that it sounded like "Unicorn". Unicorns are male more often than not, and the word "Unicorn" is male-gendered in my native language, which tipped my gender assignment all the way toward "male".

My point is, the people who are mistaking your gender may not be making any assumptions about you. They may just be making assumptions about unicorns.