Borderline Personality Disorder can seem very similar to bipolar as well, especially if the therapist isn't seeing the "relational issues" portion of their client's life, as that's where the tendencies become much more clear.
And with BPD there can be a tendency to be very open sexually, and flirtatious with many people. So I could see how a person might seem more poly inclined in that sense, though a person with BPD can have a tendency to have trouble really connecting with anybody on a deep level.
Bipolar seems to be the diagnosis du jour nowadays. And diagnosing is a tough proposition anyway as so many things overlap and people can have some traits of a type of disorder, but not necessarily have the disorder. Interesting...