Bisexuals are people with the (some include "inborn" or "innate") capacity to form enduring physical, romantic, (some include "spiritual") and/or emotional attractions to:
(1) those of the same gender as themselves
(2) those of different genders/gender presentations from themselves.
There may be an individual attraction for one gender or gender presentation which can also be fluid and changeable over time.
Bisexuality is not synonymous with being polyamorous (some include "or promiscuous"). Individual bisexual people may be celibate, asexual, monogamous or non-monogamous just as individual straight, lesbian or gay people can be.
No matter what their own gender/gender presentation or the gender/gender presentation of the person they are partnered with, bisexual people remain bisexual. They do not suddenly switch orientation as if by magic when they enter into a relationship.