Here we go, should anyone find themselves morbidly fascinated: Core, a timeless classic.
...He felt Clark's length brush his own and he reached down and grasped it. His eyes flew open and he stopped moving with Clark as he felt along the thick member in his hand. "Clark..." He whispered. Clark wanted to continue his movement but Lex's voice pulled him out of his haze enough to focus. "Clark I don't think your going to fit." He said just as quietly as before. Clark frowned and Lex tightened his hand around him, making him buck in it, and he understood Lex's concern. He had jacked off before and he knew that when he became hard he lengthened and expanded to a much bigger size than he was when he wasn't hard. On an impulse one day he had measured himself to be 15' long 5' thick.
The worry was starting to cloud Lex's eyes as he felt along the length even though he knew Clark said he wouldn't hurt him. He might unintentionally...
...Clark continued to plunge within him until he was sobbing unhindered by anything, just the feel of Clark within him, filling him up physically and emotionally deep inside his heart. He felt Clark press the small of his back again and he arched painfully as a tingling feeling spread out from that area, and he 'opened' in a unfamiliar way. Clark's last thrust pushed against something, 'into' something deep within him that started him screaming Clark's name. His orgasm was harsh and felt as if it was wrenched from his body with the feeling of gushing fluid filling him and Clark's broken cry, and darkness claimed him quickly.
...Cresi watched the readings on Lex's physiology and smiled softly. Clark's instinct led him to impregnate Lex without him even knowing it. All is going well for the future of Kryptonia's next generation.
And I'm still screaming at Lex to RUN AWAY!!! Before the mighty super strong cock o'doom rips his entrails up entirely. But I fear it's too late, because apparently Clark's cock has staked him through the heart already. Alas, poor Lex.