First of all... "Luke, the football tactics are getting into you bro"Luke, the football tactics are getting into you bro, they did hooked up. It was when Sawyer was about to die when they were held captive, remember?
He ruined the engagement because he still contacts Kate and Kate lied to Jack about it. Well, it's not ruined per se, but just take a look at Jack's face when he discovered that Kate were working for Sawyer behind Jack's back.