Marlene
This is based off of the popular headcanon that Peter, Marlene and James are childhood friends, and takes place when they are around nine years old.
First of all, James convinces the other to stage a pretend wedding (the romantic that he is), and Peter agrees pretty fast, but Marlene starts whining. She complains that it's not fair that the others will marry girls when she has to marry a boy. Ever logical, Peter asks why Marlene can't marry a girl like they can, and none of them have an answer. After putting a lot of thought into this, and not coming up with a plausible reason why Marlene can't marry a girl, James suggests they have a wedding for her and an imaginary bride. They spend the rest of the day building decorations and finding costumes to wear. The 'bride' was a doll that Peter found in James' room, which he wrapped in toilet paper for a dress. They hold the ceremony, James as officiator and Peter as ring bearer.
As they were in James' yard (the biggest of the three, where they spend most of their time), his parents are the first they talk to about their day. Effie and Monty are very fond, and when Peter asks why two women don't usually get married, Effie explains to them that two women could get married, if they wanted. (Obviously gay marriage wasn't legal at the time, but Effie and Monty were definitely allies, and they also didn't see the point in explaining homophobia to nine year old's). Marlene declares that she is going to marry a woman when she grows up, and the other two nod along. James decides that he doesn't care if he marries a boy or a girl as long as he loves them, and Peter decides that marriage is gross. (Aromantic representation was non existent, so the others chalked it up to his age).
That night, when Marlene and Peter go home, they tell their families what they did that day. They are less accepting, and tell that what they did isn't something that happens in the real world. Their parents ask if Effie and Monty knew what they were doing, but the two have enough foresight to say no.
The next day, Peter and Marlene refuse to acknowledge what had happened yesterday, and eventually James stops bringing it up. All three of them remembered that day for years afterwards, but never spoke of it again. Marlene would get anxious when gay people were mentioned, in fear that her friends would tell someone about what happened that day but they never did. In fourth year, after denying it for so long, Marlene comes to terms with the fact that she is a lesbian.
James and Peter are the first people she comes out to, and they accept her unconditionally.