Why do you think Beatrice has determined she will not marry?

Beatrice berates the institution of marriage in general terms. She goes on to say that there are too many flaws in particular men. Check out this discourse betwen Beatrice and Leonato: 

Just, if He send me no husband, for the 

which blessing I am at Him upon my knees every
morning and evening. Lord, I could not endure a
husband with a beard on his face. I had rather lie in
the woolen!
LEONATO
You may light on a husband that hath no
beard.
BEATRICE
What should I do with him? Dress him in my
apparel and make him my waiting gentlewoman?
He that hath a beard is more than a youth, and he
that hath no beard is less than a man; and he that is
more than a youth is not for me, and he that is less
than a man, I am not for him. (Act 2)