Shimmer really did make Amy Clex-coded, huh? Between her stalker behavior mirroring Clark, her obsessive shrine mirroring where Lex was slowly heading toward, and her comments about Victoria paralleling Clark's. Then, Lex telling Jeff 'that kind of love could get me arrested'; he was refering to her age, but, in a year where sodomy was still illegal in Kansas, and used to target same-sex couples far higher than het couples, in a show where age gaps and sexual creepery between het couples are hand-waved away, it definitely has a bigger meaning. I will not be convinced that Lex didn't look up Kansas' laws regarding age of consent and sodomy (which is not just anal, but technically anything beyond 'traditional sex', i.e. penis-in-vagina sex) shortly after Clark breathed life back into his lungs.
The reasons he can't be with Amy, are the same reasons he can't be with Clark. The jealousy Amy shows concerning Victoria, is mirrored by Clark prodding Lex about why he's with her. Both Jeff and Clark actively try to get Victoria out of Lex’s life. Jeff, by scaring her away, Clark by pointing out to Lex that she's spying on him.