She's stuck in the mass marketing limbo so she can't be Finn's girlfriend or anyone else's. Like the show Friends learned, when Ross and Rachel get together, fewer people care to watch. In Adventure Time's context, Finn can't get with PB or Marcy because viewers loose interest.
The show sort of winks that all the women are Finn's girlfriends.
PB is top tier. Marcy is second tier. FP is third tier. All the rest are sloppy fourths tier.
But really, Finn would fuck/prego all the girls he could get once he was about 17+ because as a legendary warrior, he would be the desire of many a maiden.