>>95621676>Until he doesn't.
I can see Star succumbing to temptation or getting caught up in a moment, I can't see Marco cheating on Jackie under any circumstance short of mind control.
Following the same pattern as Bon Bon and Starcrushed, I see the optimal setup for suffering as Star crossing the line badly enough that Marco gets really upset with her and Star ends up going home to Mewni in tears just in time for Eclipsa to make her move.
Then Marco, now feeling regretful and frustrated like Star did after their fight in Storm the Castle, somehow finds out about shit hitting the fan on Mewni and gets ready to go help and make up with Star. Only Jackie catches him right before he's about to leave, and she gives him hell because he was going to leave her on Earth for who knows how long AGAIN, without even telling her AGAIN, to go after Star AGAIN. She tells Marco that if he just can't make time for her, if he can't choose between Earth and Mewni, between her and Star, she'll make the choice for him, and walks out on him.
Once again, we get a cliffhanger with our heroes heartbroken and alone, and the villain on top of the world.Then hiatus until Spring 2018