Lex never married Lana. Back when Superman debuted in Metropolis, Lex had his scientists going over all info on Supes. Said scientists discovered that a red-headed woman was present in several Superman appearances (Lana was deeply affected by the break-up with Clark and learning he had powers, and began following him when he became SUperman). Lex had her kidnapped and tortured to know Supes' identity, but she never revealed anything.
Many years later, when Lex ran for President of the USA, his vice-president was Pete Ross, who was married to Lana Lang. When Lex went loony, Pete took office and Lana became First Lady. She later divorced Pete, and is now the new CEO of Lexcorp (Luthor was removed from the company a while ago, and putting a pro-active former First Lady as CEO was a move by the directors' board to help Lexcorp's image).