I'm not sure if it's her critical mass - he'll, we only had Star realize she's got a crush on Marco literally in the last 2 episodes. Marco hasn't even sorted through his own emotions yet and has always only thought of Star as a friend - a great friend, his best friend, but still just a friend - till now.
They've been friends for 2 seasons. Marco doesn't move fast when it comes to matters of the heart, he takes things slow and thinks things through and fears the negative possibilities. He was aware he was intensely crushing on Jackie, who was smiling and nodding at him, indicating interest - and did nothing about it for a fucking decade or so.
He's finally reached the great goal of his life; landing Jackie as a gf. Jackie's amazing, it's a dream come true. He's found a great friend in Star, they have a fantastic friendship now. It's good.
But now, romantic interest is ruining it all. He's never thought of Star as a romantic interest. It's he interested in her? If he is, it's he rrreally or it's it just confusion from being confessed to? Should he drop Jackie, just when he finally achieved the impossible and got together with her? Would their relationship with Star even work anymore if they go from friends to lovers, what if it ruins something forever between them? Wasn't Star just crushing on Oscar, what happened to that? What if it's just a passing thing, she was influenced by the song, and he'll lose both Jackie AND Star if he leaves J for Star and she changes her mind or he disappoints or bores her? Even Tom couldn't hold her interest for long, and he's a handsome prince with magical powers. And doesn't her bf have to be a prince anyway?
I really don't see him deciding to damn it all, leave Jackie and decide to pursue Star romantically within a few episodes. It's quite ooc and would be rushed hack writing, frankly. If anything, he'll try to make things go back to the way they were