Here's an answer from someone that is Bisexual, yes we are attracted to both sex, but that doesn't mean you have to have both sex for your partner, I've been with my boyfriend for 6 years now and still can say that I'm attracted to both sex, I've dated 3 different girls and 3 different guys. Basically it is a preference, just like if you were straight, for example I'm a girl that likes dark hair guys but then again I also like guys with blond hair and blue eyes. Its just a preference. Also allot of people tend to think bisexuals cheat allot because people think because they like both sex its making them extra horny, but that's not true. You could be a cheater if you where bi, gay, lesbian or , straight it doesn't matter. I'm strongly against cheating and have not ever done so.
It normally is understood to mean a person is sexually attracted to both sexes.
It is actually also within the mental illness category generally from the parental dysfunction and confusion from bad parenting