Coming out to denying mother.

Tell her that such a thing exists. Say that you can date either gender. And tell her what your stance is on sexual preference and how you feel towards each gender.

 

I realize that we are talking about your mother and she is probably a fine person who only wants the best for you.

However, my reaction is: How the Hell would she know?? This is almost as bad as the It's A Choice rhetoric. Anyway, this will be an educational process. There are many books and articles about bisexuality. Distinctly, it does exist. Only slightly tongue in cheek, it's like being gay with greatly reduced constraints regarding genitalia.

So, before you can correct your mother, you need to do some homework regarding things to read. Once you get that sorted, give them to your mother...

 

Try to explain to her that attraction is a spectrum, and you can be anywhere on it (even off of it). You say you never have sexual attraction to girls, that's biromanticism, and romantic attraction is also a spectrum, so you can definitely be biromantic and heterosexual, or biromantic and bisexual with out wanting to act on any sexual desires you may have.

I agree with flatlander, its a good idea to educate yourself so you know what you're talking about, and can say it confidently. Also its nice to understand yourself better, I find

Good luck!