I think she was just a lot more into the whole 'hungry and horny' thing than Buffy was and wasn't particularly concerned with who her partner was. Hypersexual, rather than hetero/homo/bisexual. Faith is also quoted as having borderline personality disorder by the Wiki article on her (which I assume came from some interview, comic, or printed material that's not on the show), which adds to the hypersexual idea. "People with BPD exhibit other impulsive behaviors, such as excessive spending, binge eating and risky sex. BPD often occurs together with other psychiatric problems, particularly bipolar disorder, depression, attention deficit disorder, anxiety disorders, substance abuse, eating disorders and other personality disorders."
A bit more on BPD...
While a patient with depression or bipolar disorder typically endures the same mood for weeks, a person with BPD may experience intense bouts of depression, anxiety, or anger that may last only minutes, hours, or at most a day. These may be associated with episodes of self-injury (including cutting), impulsive aggression, and drug or alcohol abuse. Difficulties in cognition and sense of self can lead to frequent changes in long-term goals, career plans, jobs, gender identity, friendships, and values. Sometimes people with BPD view themselves as fundamentally bad or unworthy. They may feel unfairly misunderstood or mistreated, bored, empty, and have little idea who they are. Such symptoms are most acute when people with BPD feel isolated and lacking in social support, and may result in frantic efforts to avoid being alone. Ironically, it is the desperate clinging to other people that often serves as the very catalyst for conflict that drives others away.
I know that Whedon was intending to turn Willow or Xander homosexual during season 4 and so season 3 was riddled with stuff like Larry and Xander, or VampWillow's bisexuality just to see how people reacted to the idea that each was gay. I wonder if Faith was introduced originally for Willow/Faith if they took the lesbian route, but was then sacrified due to an inability to contract her for long-term use on the show? Like how we got Andrew in the AtS S5 episode with Dana, just because nobody else was available? They couldn't secure Eliza for more than a solid season, so she was written out at the end of S3, and popped back in here or there until the tail end of S7?