It's still a hot hookup. Pun not intended. Besides, they can't put Finn with the top tier girls (PB, Marcy) for any length of time because then they loose the shipper audience.
Hell they could do a number of plots where Finn gets with one or both of them.
E.G.: (Skeleton Plot) Finn accepts a job as night watchman for Candy Castle because spirits have been haunting it lately. After a few weeks of regular duty and some prayers to Glob, the spirits have gone away. PB is overjoyed and kisses Finn late one evening, for his faithful service. The next scene is morning and Finn and PB wake up next to each other without clothes on and she smiles and kisses him again. "I've got to get up babe she says. The Kingdom doesn't run itself."
Or a plot with Marcy: Finn suffers a wound on his soul in a battle with a necromancer who he manages to slay. Marcy upon hearing that Finn has been hurt so badly, promises to torment the Necromancer in the Nightosphere for all eternity... but first she will heal Finn's soul? (She is the daughter of the devil and used to farm mortals for food. She knows how to fix their bodies and souls just as well as she knows how to kill and eat them.)
So she takes Finn into a private room and won't let anyone come in for several hours. When she and a much rejunivated Finn emerge (arms around each other,) their clothes are on loosely, she has a hicky on her neck where she was bitten and both her and Finn have messy hair.