A lot of people with BPD are also victims of sexual abuse (but not all people with BPD were abused, sexually or otherwise.) Sexual abuse has a known link with hypersexuality (not all that are abused become hypersexual.)
However, BPD is associated with a difficulty with boundaries and a deep willingness and capacity to love - which can be expressed through sexuality... and that would be linked with the 'favorite person' and how the splitting is at any one time.