Look it up in the archives. First of all there's that Star is obviously based on Nefcy and Marco is obviously based on Bobby. Then that Jackie was based on a girl Bobby had a crush on. Nefcy went out of her way to pair up Marco with other girls and girls are constantly flirting with him. There's also Star's horns, horns are a symbol of being a cuck. Then you look at the other things Nefcy has done. She admits to being a voyeur and her voyeurism often appears in her art:
Check 2:28 in this video.https://www.youtube.com/watch?v=vGWgiJ0aDy
One of her early cartoons was about a girl getting kidnapped and put in bondage and then when her boyfriend goes to rescue her he gets seduced by another woman and make out with her in front of the kidnapped girl:https://www.youtube.com/watch?v=fLa-9yqgdFE
Then you have Nefcy doing the voice for Starfan13, who has an unrequited lesbian crush on Star, so she is actually cucking herself. In the Starfan13 live chat where Nefcy was adlibbing Starfan13's lines she talked extensively about having a crush on both Star and Marco, wanting to be invisible so she could spy on both of them, writing steamy fanfic about them, etc.
There is a lot more but look it up yourself.