Remus and Sirius had been secretly dating since 5th year.
It was the first quidditch match of the season & Gryffindor won. There was a massive party in the common room celebrating this, Remus wasn't one for parties and only stayed there briefly to congratulate his best friend on the achievement then went up to bed to read his book. Sirius followed shortly after, claiming he was tired but really, he wanted to be with his boyfriend.
When he got up to the dorm that he shared with his 3 best mates, he walked over to Remus' bed and sat there until he finished his page (Remus was already close to the end of it). When he looked up, he saw Sirius sitting on his bed and looking at him in a loving way and then asked "any book recommendations". Remus reached down, into his trunk and reached out a book that he knew Sirius would enjoy. Sirius wasn't one for reading but as soon as he and Remus got together, he started to ask constantly for book recommendations and began reading together whilst sitting close together on one of their beds. This would even happen when Remus was in the hospital wing, recovering from a horrible transformation to try not to look too suspicious when all he wanted to do was be with his boyfriend.
Around an hour passed, the party had died down, James and Lily walked up to their dorm and the minute they stepped in, Lily started to cheer "I knew it, I knew it! James, gimme the 10 gallons!" James unwillingly gave her the 10 gallons and sulked in defeat.
Over the course of that hour, Sirius and Remus had unknowingly drifted closer together to the point they were almost on each other's laps. They realised this the minute Lily asked James for the 10 gallons.
They jumped off each other, Remus nearly falling off his bed. Sirius asked, "you were betting on us?" James replied with a nod then turned to Lily "1:1 now, we need to sort out a decider on who's got the better gaydar"
Remus, thankfully not injured, asked "who else did you bet on and since when?" Lily replied "we also bet on Marlene and Dorcus, James won that one" "We've been betting since we got together, back in October" James explained "I mean, I have been trying to figure out since 5th year why you would only read when you were with Remus"
Remus asked, "Do you think Peter will accept us?"
Lily replied optimistically "of course he will, he's your best friend after all"
They found out about Marlene and Dorcus a few weeks prior. Lily was packing her stuff to go and spend the night in James' dorm for the first time that they had planned. Some people would say that they moved too fast in their relationship, but they didn't care because they were in love.
She was grabbing her PJs with the thought in the back of her mind of potentially stealing some of his clothes but grabbed them just in case. Marlene and Dorcus were helping each other with 'homework' but really spent the majority of the time talking. Lily was just about to head out when they began to kiss, she quietly grabbed 5 gallons from her wallet and went down to the common room to meet with James. She told him that they'd kissed and begrudgingly handed him the 5 gallons.
They went up to the boys' dorms for the evening.
Lily did end up 'borrowing' one of James's hoodies but never gave it back.