Kate: Because as corny as it's going to sound, LOVE is more vital than lust. LOVE is the driving force behind every action Elizabeth and Will take for each other.
There is no way that Elizabeth was going to sacrifice Will for Jack. The writers note several times that "the only way to have Jack and Elizabeth kiss was to save Will", and that there isn't much debate about "how much Elizabeth weighs Will's life" when she makes the decision to chain Jack up. Will, Will, Will, Will....everything revolves around Will.
She wasn't going to shoot the barrels when Will encouraged her to do so, even though he knew he'd probably die. She wasn't going to shoot. She didn't want to...yet there was only a brief hesitation when she chained Jack up.
Maybe, in AWE, TnT can convince me that Elizabeth's love for Will has faded from being able to kill for him to leaving him for Jack. They're talented writers, it could possibly be done.
But so far, the argument of J/E look hot together does not stand up against the fact that Elizabeth killed for Will.
LUST, which is what TnT have mentioned as what Elizabeth is feeling, tends to be fleeting. Lust is typically considered unhealthy, shallow and self-serving. Which is why Jack ends up chained to the mast while Elizabeth goes off with the man she loves -- Will.
Lust is important, and it's powerful, but nothing can compare to deep, real love. 😄 That's the answer to your question, poorly worded though it may be.