Im just getting specific in a reply to the commentor above.
A bisexual + “plus one” implies that the plus one is not the regular partner, and that if the “plus one” is not bisexual, i.e in this context engaging in sex with the same gender, then that implies the original couple is instead.
So a threesomes is not a “bisexual, plus one” unless the couple is in a same sex relationship, i.e 2 men or 2 women who invite a 3rd of the opposite sex to bed.
A threesome with an otherwise hetro couple who invites a 3rd of either gender to all have sex with is a bisexual threesome, but not a “bisexual, plus 1” situation. The same threesome where everyone engaged in only hetrosexual contact would just be a hetrosexual threesome.