Princess Bubblegum = The creators weren't expecting a "love relationship" going on in the beginning of the series. This প্রদর্শনী is called Adventure Time, so they wanted to start off with adventures and not lovey-dovey stuff. Anyways, a 13 বছর old hooking up with a 18 বছর old wasen't really age appropriate I guess.
Marceline = I think Marceline was supposed to be আরো of a challange for Finn and Jake, not a প্রণয় intrest. Anyways, Marceline dosen't appear in alot of episodes anymore, so we cant learn much আরো 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.
Marceline = I think Marceline was supposed to be আরো of a challange for Finn and Jake, not a প্রণয় intrest. Anyways, Marceline dosen't appear in alot of episodes anymore, so we cant learn much আরো 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.