From the first time Sam saw Lucifer, he knew he hated him, but his vessel was immensely beautiful, he couldn't deny that. Then after he was back in the cage, and Castiel had taken his pain and craziness, Lucifer returned in his head. Sam knew Gabriel wouldn't be happy that he actually had conversations with Lucifer when he wasn't around.
Admittedly, Sam didn't hate talking to him, he wasn't as annoying as he had been previously. The conversations were deep, meaningful, Lucifer had even told Sam a few stories about Gabriel in heaven. Then Lucifer started to advance, he crossed the line, appearing on Sam's lap one day. Sam had been pretty surprised, when Lucifer had kissed him, and not just a little kiss, this was a full on French kiss.
From then on things escalated, from just making out to Lucifer groping him when he felt Sam hard against his thigh, and things escalated again after that, to the point that as soon as Gabriel left they'd end up in bed and if Sam didn't know better, he'd think Lucifer was real, the way he could romance him and make his heart pound with just words.
Things started to get more complicated when Gabriel began to notice how tired Sam was around him, and how distracted he was. He confronted Sam about it at once "Sam what's going on with you? You're always tired and distracted.." said he. "Nothing.. I've just been talking to someone a lot.." Sam returned with a shrug, Gabriel looked at him "Who?" he asked. Sam scratched the back of his neck awkwardly, "Lucifer…" he mumbled. Gabriel's face was one of shock, but he didn't say a word, he just disappeared, Sam knew he wasn't coming back, not anytime soon.
Lucifer was there of course, he was there the nights Sam cried himself to sleep, the days that seemed to last forever without Gabe, even being around Dean didn't help. Eventually, Gabriel sent a letter, it read
Sam,
I love you like there's no tomorrow but what you've been doing with Lucifer, my brother, I can't forgive you for. Sorry Sammy, its over, for good, don't bother praying, I'm not going to answer.
Gabriel.
Sam was in tears reading it, but when Dean asked him what was wrong he just shrugged it off, always did. The day Sam got the letter is also the day he and Lucifer got hot and heavy. As soon as Sam was in the door, Lucifer had his lips smashed against the hunter's, groping at his hard length through his jeans. Sam all but growled "Clothes…gone…now" he said, and with a snap of Lucifer's fingers all clothing was gone, and Lucifer had been pushed to the bed, Sam's mouth instinctively going for his member but Lucifer stopped him, "Not this time baby, this time, I dominate you" He smirked, snapping his fingers again and Sam was bound to the bed with rope. Lucifer being mischievous had added a little surprise, a cock ring around Sam's erection, the younger male groaning at the pressure. "Dammit Luci! Not in the mood for you to torture me right now… Need you!" Sam groaned, Lucifer had also now added a blindfold, "Sammy… my impatient little slut" he said in a voice that was velvet over steel "I'm not going to tease you, I'm just tying you up because I like having the advantage" he smirked as he kneeled between Sam's legs, fingers lubed up and one sliding easily into Sam. He prepared his hunter for some minutes, before he slid his own cock into Sam's entrance in one quick movement. "Move it Luci! Don't just sit there!" Sam moaned. Lucifer was already close, but hearing Sam's words he came there and then, quickly removing the cock ring from Sam's member before he did. They came together and after, they lay there, in each other's arms, before Lucifer spoke "So… I'm sorry about you and my brother, and… uh.. Sammy… I'm not actually in your head…" he said casually.