I don't think that the sexual attraction to men and women needs to be equal.
Just as being straight doesn't mean that you're attracted to all members of the opposite sex and being gay/lesbian doesn't mean that you're attracted to all members of your own sex.
Having some sexual attraction to those of the opposite sex and some attraction to those of your own sex would constitute bisexuality....whether it be equal or not, and certainly not requiring it to be equal at all times.
There's a thing called "sequentially bisexual" (a term I found out about in a research study) where one is sometimes more attracted to the opposite gender and sometimes more attracted to their own gender.
I thought of myself as straight until I first had sex with a man, after which I considered myself bisexual for many years. In recent years, I've realized that I'm not sexually attracted to women, but only to men....so I now think of myself as gay.