Clarke is surprised but also tempted, and lets it slip that he's had thoughts of that nature. Keira asks if he wants anal sex, and wants HER. He hesitates. Keira points out that Clarke said it was okay to dream and fantasize about these things, right? Then he should pretend this is just a dream. What would he do if there were no consequences? What would he do to her THEN? He looks at her with desperate yearning, and admits he would do anything and everything to her. It's clear that Clarke's resolve has faded, and that he's ready to give in completely to Keira. They have sex, including the anal exploration they've both been hungering for, and even an anal creampie.