Bisexuality is romantic attraction, sexual attraction , or sexual behavior toward both males and females ,    or to more than one gender. The term bisexuality is mainly used in the context of human attraction to denote romantic or sexual feelings toward both men and women,    and the concept is one of the three main classifications of sexual orientation along with heterosexuality and homosexuality , all of which exist on the heterosexual—homosexual continuum. A bisexual identity does not necessarily equate to equal sexual attraction to both sexes; commonly, people who have a distinct but not exclusive sexual preference for one sex over the other also identify themselves as bisexual. Scientists do not know the exact cause of sexual orientation, but they theorize that it is caused by a complex interplay of genetic , hormonal , and environmental influences ,    and do not view it as a choice. Bisexuality has been observed in various human societies  and elsewhere in the animal kingdom    throughout recorded history. The term bisexuality , however, like the terms hetero- and homosexuality , was coined in the 19th century.
A New Sexuality: Not Straight. Not Bisexual. Mostly Straight | Time
However, when I am physically intimate with a man I find it difficult, if not impossible, to maintain an erection. When people are grappling with such questions, what they are really comparing is not so much the qualitatively different sexual experiences, but rather who they experience themselves to be in the context of their relationships with people of different genders. But you do not have to make a choice — not now and not ever. You accept that you are bisexual , so you can fairly comfortably decide to have relationships with people of either or both genders.