Not everyone feels it's appropriate to share every detail of their past with their significant other.
If the other person is a guy, though, I think once you become intimate and start sharing what turns you on, the interest in other women would come up if you're bi.
I suppose intimacy comes in many forms but intimacy, by definition, is sharing your innermost private thoughts, emotions and actions with another.
I know women who have chosen not to share their bisexuality with their male significant other and although they manage and probably have great relationships, their intimacy is not as deep because they choose to leave that out of the things they share with their partner.
As far as being "out" to everyone like friends, family and neighbors, at work, etc., unlike lesbians, who, in order to function normally in society and have relationships, must at some point, tell their loved ones what's up, the bisexual woman can keep it to herself and close friends or whatever, until or unless she chooses to have a long term relationship with another woman. There is no fighting for equal rights, etc., for the bisexual woman in a relationship with a man.
I think it's probably easier in life to deal with everyone if you do eventually "come out" to your family, but if the situation doesn't ever present itself where it is necessary, there's no impetus to do so.