I could seriously do without the whole love triangle. It's just too Grey's Anatomy for me. I liked Kate better when she was just looking out for herself.
But, since it is part of the show and I am a fan of the show, I offer up these comments:
Jack's already been through one instance of falling in love with someone and seeing her with another man.....I think his decision to cut Kate loose is a good one. Kate is torn up about Jack being with Juliet not only because she has feelings for him, but also because she sees Jack to be a stand-up, moral guy. So, if a guy like Jack loves her, then she can't be a bad person like the other people in her life have tried to make her out to be. Being rejected by him, then, is a double blow.
I agree with Jugdish that Sawyer pretty much goes into the "win" column just for getting to sleep with her. BUT....every guy knows that there's a Catch-22

there if you really care about the girl. If you really don't give a damn about her, then revenge sex is great. If you do, however, then it may not be such a good deal for you to be the rebound guy, because once the girl gets over Guy #1, she may start looking at you as the guy that took advantage of her in her moment of weakness, or she may decide that she only slept with you to get back at her boyfriend and not because she really liked you. Either way, you may have blown the chance of a continued relationship.
In short, if you don't take the opportunity to have sex with her this one time, you may not get to have sex with her at all. By TAKING the opportunity to have sex with her this one time, you may lose the opportunity to have sex with her a bunch of times.
Island Women .... Can't live with 'em, can't kill 'em and expect them to stay dead.