Princess Bubblegum = The creators weren't expecting a "love relationship" going on in the beginning of the series. This show is called Adventure Time, so they wanted to start off with adventures and not lovey-dovey stuff. Anyways, a 13 year old hooking up with a 18 year old wasen't really age appropriate I guess.
Marceline = I think Marceline was supposed to be more of a challange for Finn and Jake, not a love intrest. Anyways, Marceline dosen't appear in alot of episodes anymore, so we cant learn much more aboout her past such in the episode "Memory of a memory", where we see Marceline with her ex, and her with her dad at the restraunt as her dad was eating her fries. Anyways, Marceline would never hook up with Finn.