Marauder's Era
Lily and Remus have a night together, maybe after drinking, at sixteen/seventeen in their Sixth Year. Lily gives Percy up for adoption, deciding to keep it a secret after Remus reacts badly to the idea of children (which is canon, and I imagine that a teenager would react worse than adult Remus did). He also pulled back from Lily somewhat because of James and his feelings for Lily. Dumbledore may or may not be aware of the events depending on how Lily hides her pregnancy, as well as how manipulative Dumbledore turns out to be. Molly, Arthur, Bill, and Charlie know that Percy’s adopted, but no one else. None of them know who his parents were because it was a closed adoption. Lily will have a hard time with this and James will be a surprising source of comfort for Lily during this turbulent and lonely time post pregnancy. This can be the start of them getting together in their seventh year. Lily can be more distant from Remus because of what happened. Lily may or may not tell James, but either way, he can pull back some from Remus after the wedding because Lily has. This distance could be part of the reason he’s suspected as a traitor and left out of the loop with the Secret Keeper plan.