Steve and Robin bumping into each other when they were oggling Eddie and Chrissy in high school. Steve panics, thinking she's caught him, and accuses her of looking at girls first. She counters that he was looking at boys.
It takes a while but, eventually, realising that they're both in the same boat. They decide to fake date as a cover for both of them, trying desperately to come across as straight and "normal".
Eddie, who had already found out about Robin after a high night together, is a little too protective and tries to confront Steve about it. He wants to make sure she's safe and not at risk because of her association with him.
It devolves into Steve panicking and admitting to his crush on Eddie in hopes of protecting Robin, trying to keep her from being outed... which only endears him to Eddie, who is immediately full of heart eyes.