That’s what you get for commiting genocide

More or less, most girls in the show dated/liked the main male character Marco at one point or another throughout the show.

Whereas the main girl character Star gets upset/jealous over it several times, as well as some fans generally disliking her character for various reasons, especially towards the end. So they often have fun portraying her as a the CuckQueen.

The trap aspect just comes from a ongoing plot-point of Marco having to dress up as a girl to rally Magical Princesses against one of the show's antagonists. Though the show never really takes a romantic angle with his trap-persona.