Headcanon 5
After the events of "The Plasma Monster", Booster and Petra became good friends and accepted the fact that Petra in a relationship. In fact, he supports them now and they appreciate it alot. He even befriended Plasma Boy as well, and they were cool after what had happened.
One time, Petra even invited Booster to her birthday celebration, he knew exactly what to give her and as a birthday present and he bought her an autographed vinyl of her favorite rock band, which she was absolutely thrilled about and thanked him for one of the best birthday gifts she's ever had. Booster was happy that Petra was happy with her gift that he worked so hard to find for her.
As her friend, Booster is willing to be there for Petra when she needs him, support her and will listen to her problems without judging her. Petra is grateful to have such a good friend like Booster.