This article is from the BiSexuality FAQ, by Jon Harley firstname.lastname@example.org with numerous contributions by others.
Not by any useful definition. A useful definition of bisexuality might be,
anyone who has serious relationships with members of both sexes, and anyone
who identifies as bisexual. It is possible to suggest that everyone has some
potential for attraction to both sexes, but since most people(*) never act on
it, this is pretty irrelevant.
If someone says that they are straight, or (gay/lesbian) then for you to
insist that they are "really" bisexual but perhaps just don't realise it is to
deny them their self-identity. Everyone should be free to define their own
identity for themselves, which invalidates this kind of generalisation.
Moreover, bisexuality is not BETTER than being straight or gay. The best
thing for each individual is to be what they feel is right. So please do not
think that people identify as bisexual if they are "more highly evolved" or
more in touch with their inner feelings. Accept diversity - different people
really are different.
(*) Research carried out at the Harvard School of Public Health, USA in 1994
found that 20.8% of the men and 17.8% of the women studied admitted to same-
sex sexual attraction/behaviour at some time in their lives.