Marco, the male deuteragonist, was already quite in love with a girl, Jackie, the show made a huge emphasy on making it clear that it was a strong love and more real love than usual "teenage crush".
From episode 2 Star, the protagonist, noticed Marco's love for Jackie and tried to help him during at least 3 episodes, most of the time she fucked up but as she was the main character, something always happened and her mediocre plans worked at the end, during the whole thing there was not a single time where she seemed interested in Marco other than trying to help him as she already had a crush on a rockstar wannabe guy she met during an episode.
The moment Marco and Jackie began dating, and during the next episodes that had them together Star was always depressed and suffering, this was a huge reveal as Star seemed to push onwards on making Marco and Jackie a thing and everybody began wondering if she was a Cuckquean. It didn't help that Daron, the creator, had a video where she mentioned she was a voyeurist, since Star was so easly hateable people began wishing Marco was cucking her constantly as other girls in the show began showing interest in Marco. With the finale forcing the two together the desire for Marco cheating-cucking Star with the other girls only increased.