I don't care that Finn never ended up with PB or Marceline. Anyone with a brain would realize Finn would grow out of his puppy love for PB and that she was never end-game. Marceline and Finn had a better chemistry and she teased him a lot but that one was also probably too unlikely to end on.
I don't care that PB and Marcy getting together completely neutered whatever character they both had to turn them into Tumblr's ideal of two chill, perfectly reasonable lesbian moeblobs mushing together.
I just can't stand that for all the arcs and episodes about romance and dating, Finn doesn't end up in love with anyone at the finale. Flame Princess and Finn had their issues and arguments but were a solid couple for a while, but FP dipped out to rule her kingdom with Cinnamon Bun. They tried to force Huntress Wizard in around the end but then in the finale they don't share a single scene together. Finn doesn't end up alone, persay, but that they had seasons worth of content exploring Finn's romantic life only to end on fucking nothing is astounding to me.