I'm with you pureshipper. Finn and Bubblegum were very forced. I don't think Bubblegum ever saw and still doesn't see him as her equal, and for someone her age, I believe that's important. Even when she deaged herself, I don't think she viewed him as anything more than a play companion.
Now Marceline is someone who has lived as long as her and constantly challenges her. Bubblegum needs someone who will call her on her B.S. and I think deep down she knows this. All her candies are 'yes ma'am' types. Even Finn doesn't ever question her.
Bubbline forever!