This one turned ugly.
Smile Clark/Lex G
"You're too old for him." Luthor stood next to him on the Gotham City rooftop, arms bound in long, black cords with bats on the ends, and smiled. Clark hated that smile.
"What are you talking about?" He turned back to watch his partner of the moment leap over roof tops and around gargoyles in the moonlight before he faded back into the shadows.
"Him. Batman. *Wayne*." Clark turned, making his eyes flash blue, hiding the fact that his blood ran cold at those words. Luthor knew. "He likes little boys in tight pants, even if he won't touch them." The smile increased. "Sexual frustration does look good on him, though. Gives him extra righteousness, and Lord knows he needs all the righteousness he can get."
"You just shut that mouth of yours, Luthor."
Luthor just smiled again. That same hateful sneering smile he'd developed right after his father died, the one that made him look just like his father. "Hit a nerve, did I, Mr. Ken" Clark jerked. "Oh, I've known since the beginning, Kent. I've known you since you were fifteen, and I've watched that body of yours since you first entered my life. Those gaudy tights of yours just make it more clear." He stood on his toes and kissed Clark's startled mouth. "Just as soft as I'd thought. You have a mouth for kissing, Clark. And for blowjobs. Are you good at blowjobs?"
Clark stared. He'd played with frat brothers in college, and there was a night he'd spent with Nightwing after Dick left Bruce for good, but there was that first time behind the castle, in the rose garden, Lex hot and smooth on his tongue, his own cock harder than it had ever been - hard enough to make Lex scream later on. His going away present before going to Metropolis, with a promise for more, except that Lionel had died and that smile had appeared.
"Who are you? Where is Lex?" His mind cringed away from what had to be the truth, but Clark was a reporter. He could not ignore the facts.
"I suppose it *is* an uncomfortable notion, isn't it?" The smile broadened. Clark shuddered. "Lex is dead, Clark. We buried him under my name, which was fitting because he was in my old body. He did fight me every inch of the way. You'd have been proud of him. I know I was. But he was flawed and not fit to take over for me."
"He was twice the man you are - and he would have been even greater if you hadn't broken him in the first place."
"Ah. I see, despite the lovely Miss Lane, that you still love him. How...futile, as love always is." Lionel Luthor smiled. "And no one will believe you." He kissed Clark again. "Such a beautiful mouth. My body is the same - you enjoyed it once. I could allow you the same...privilege again. Or I could tell the world who you and Batman really are. Oh, there are so many things I can do." Lionel smiled.
Clark had pledged a long time ago to never kill a human. But Lionel Luthor was a monster, and Clark knew what to do with monsters. There wasn't much left of Lex's body when Clark arrived in Metropolis after flying very, very fast from Gotham.
The next day, Superman broke up with Lois, and disappeared, while Clark Kent began laying flowers on a grave that said Lionel Luthor.