It is not easy to determine how common bisexuality might be since little research has been done on this subject. 

Based on research done by Kinsey in the 1940s and 1950s, as many as 15-25% of women and 33-46% of men may be bisexual, based on their activities or attractions.

Bisexuals are in many ways a hidden population. In our culture, it is generally assumed that a person is either heterosexual (the default assumption) or homosexual (based on appearance or behavioral clues).  Because bisexuality does not fit into these standard categories, it is often denied or ignored. When it is recognized, bisexuality is often viewed as being part heterosexual and part homosexual, rather than being a unique and complete identity within itself.

As a result, bisexuality is perceived to threaten the 
accepted way of looking at the world by calling into question the validity of rigid sexual categories.