They're not going to. In the situation they're in right now the only way to move the narrative forward is if Marco realizes his feelings for Star.
Think of it this way: in Jackie's shoes, would you be ok if your girlfriend stays friends with her close friend who just confessed to her in front of you and your friends, who also happened to have lived with her? Adding to that, Marco already knows Star likes him so he can't pretend to be ignorant about the topic. This show is about choices and consequences, and Marco will make his.
Sleepover already gave us the fuel for Marco to change his feelings from Jackie to Star after that confession.