Forced bi refers to a practice common within the BDSM community where a dominant forces a submissive to engage in sexual activities with someone of the same sex as the submissive. In these situations, the submissive is bisexual.
Kinkly explains Forced Bi
Forced bi is most commonly seen in BDSM relationships where the dominant is a female and the submissive is male. However, it can be a practice that occurs in any BDSM partnership. The forced bisexual activity is a form of demonstrating control and submission as well as a form of humiliation.