"You—"
She kissed him to shut him up. She knew he wouldn't agree with what she said, but she is cowardly and greedy. She does not want to commit and does not want to reject him at the same time. Thus, a relationship without a label, that they would let run its course was the only thing she could give him.
Jess was kissing him but he stood there numb and confused. He clearly said his intention, why would Jess choose an open-ended relationship. They are not high school kids for Pete's sake!
He let her kiss him but he didn't kiss her back. He knew he was acting like a jerk, but he does feel insulted that Jess didn't believe or trust what they have. They both like each other, they're compatible in bed, where is the problem?