Kurt is a teenage model and very famous, but the industry is very dark and he is struggling not to let himself drown. He has an eating disorder and everyone around him is really messed up. He's also sleeping with Blaine, who is much older and his manager or something and who basically discovered him.
Blaine knows the relationship is unhealthy and he feels so awful for having sex with Kurt, but he keeps doing it because he needs to. He also has a lot of guilt over bringing Kurt into the industry to begin with and also has a cocaine addiction. I think he also refuses to have anal sex with Kurt because he wants there to be one first time he doesn't take from him and so he can have a first time with someone he loves. But then Blaine eventually does that too.
I remember a scene where Kurt goes and hangs out with some models and they're all getting high and Kurt is just so depressed. There's also a scene where Burt finds out about everything and flips out. In the end I'm pretty sure Blaine and Kurt run away together.
Thanks! (And I'm categorizing this as rape/dub-con because the age difference could legally be statutory rape depending on where you live, fyi )