Sexual orientation is a large part of online dating; as such, a whole plethora of acronyms have emerged.
BIF stands for bi-sexual female.
Bruce had been cruising online dating websites to find a third for his polyamorous unit. His current partner, Lacy, really wanted to find another woman who was bisexual, as she was curious about that dynamic in their relationship. Wanting to narrow his search, he decided to put in 'Looking for BIF' to make sure that anyone who reached out to their profile understood what they were looking for.