Feelings
James always seems to end up here—in bed with Sirius. He knew it was wrong, especially now, when he's married to Lily. But he couldn't help it. Sirius was both familiar and unfamiliar at the same time. There was a feeling of excitement whenever he was with Sirius. And Sirius knew what he wanted; Sirius always knew. So, James kept coming back—to the familiar of Sirius' body against his, to the familiar feeling of Sirius inside him, to the ever changing experience with Sirius, to Sirius and his undying love. James couldn't stay away from Sirius, even if he tried, even though he is hurting the people he loved most; hurting Lily by always going to Sirius, hurting Sirius by continuing coming, giving him hope that James would stay this time. James knew this was wrong but he couldn't help it. So he always ends up here in bed with Sirius.
Sirius could never say no to James. It was impossible for him. So ever time James showed up, wanting him, Sirius let James in—because he wanted this as much as James did. Despite the hurt the next day, when James left to go back to Lily; he always did. But still Sirius let James come to him. Because Sirius wanted James—wanted to be inside of him; because James always made him feel safe and wanted. And being with James, being inside of James, sucking James, made Sirius forget the raging war outside; made it feel like the world would be ok, just for a moment. A moment that always ended with James returning to Lily, leaving him alone. Despite this, despite knowing this, Sirius always let James in. Because Sirius could never say no to James.
Lily knew. She knew where James went. She knew what he did with Sirius. She knew what he really did the nights he spends at Sirius' instead of with her. And it hurt. It hurts so much, knowing that her husband wanted his best friend more than her. But that wasn't what hurt the most. What hurt the most was that he comes back to her, showering her with so much love, pretending he wasn't cheating on her. It hurt that he couldn't admit what he was doing to her. She should leave; she really should leave him. But she loved him and they had a baby coming. A baby, he didn't know about. Maybe if he did, he wouldn't go to Sirius. He would just stay with her and stop this. Stop hurting both of them. Because Lily knew that this hurt Sirius as much as it hurt her. But Lily still let James come back to her when she really shouldn't. Because Lily knew the truth.