The theme of Heat was not sex. Okay, it *was* sex. It was masturbation and heteroeroticism and more homoeroticisim in the dialogue and acting than could be explained by mere chance or actorly interpretation. Not to mention, a dozen other obvious sexual metaphors just for fun.
But that wasn't *all* of it. Underneath the flames and the flaming, there was also the issue of trust. To be precise, of Lex Luthor's desperate need to trust someone. Example, in the poolroom scene, during the Lex and Clark confrontation., Lex doesn't seem hurt that Clark was attracted to Desiree. He's disappointed but not upset that Clark "came on to her." What hurts him is that Clark is, from his pov, *lying* to him. Because he wants to trust Clark, he *needs* to trust Clark - he wants to do it so much that he's willing to cast Nixon off in the face of massive proof of Clark's lies. I think he was relieved that night by the fence when Clark was injured.
And it makes sense that Lex wants someone he can trust - he's been betrayed most of his life. His father won't let him lean on him; the very sky attacked him. His baby brother died, his mother died (and to a kid, that's a hell of a betrayal), and Pam went away. His *hair* deserted him. He's dying for someone who isn't in it for his money or influence.
My goodness, if Jonathan Kent had wanted Lex Luthor out of their lives, he could have done nothing better than *accepting* that truck. Lex would have been assured that the Kents are just like everyone else and gone his own way. But no. He can't let Clark take the reward. The Kents, therefore, are *different* and special and he wants their trust and their approval. But more than that, he wants to trust their son. Not just *love* him, although I believe he is in love with Clark, but also rely on him to be honest, to be the thing in his life he could rely upon.
It goes further - Lex's major reaction to Desiree is a need to trust her. To believe her when she says things that are not true. That was her real pull, not her pheromones. It didn't go away until first Clark and then Martha mad Lex *think* about what was going on. And he trusts Clark and he trusts Martha, and that probably saved his life.