Kurt's grandparents have a beach house where he spends every summer and the Andersons move in next door. The first summer is right after Kurt's mom dies and he's very sad and hiding from his cousins when he and Blaine meet. Kurt and Blaine become friends and eventually fall in love. They had a special place ... a cave? ... where they hung out together and I think that's where they had their first time as teenagers. The story ends with them coming back as adults (maybe with their own children?) because Kurt's grandparents have left them the beach house.
I read a fanfic a while ago where kurt and blaine were together (everyone still in high school) and they started to have threesomes with quinn. The first one starts just with her watching them then she joins in, they also all hook up at a party and the last chapter I read ended with Puck I think it was walking in on them. If I remember correctly it was mostly smut focused rather than an actual plot. Anyone know which one I'm talking about? Thanks!