If you are bisexual you feel sexually attracted to both guys and girls.
The choice of a mate is based on personality and has less to do with the other being male or female. Bisexuals can have a relationship with a girl at one time and fall in love with a guy the next time.
Establishing a relationship can be hard. A bisexual living with a guy may miss the (sexual) contact with a girl and the other way round. For some bisexuals making a choice implies loss. Some of them try to solve this by having two relationships at the same time. But many bisexuals are monogamous.

I like girls, but sometimes fantasize about guys. Will this change when I get older?
I’ve a relationship with a girl but also sex with my best friend. Is it bad to be bisexual?
Is it normal if I have a boyfriend but to want to make love with my girlfriend as well?