The First Order captures Leia and Kylo is tasked with interrogating her. He's always loved his mother a little too much, ever since he was a child (maybe it was part of why he was sent away?), and during the process of her trying to talk him out of being the second coming of Vader he takes his helmet off and starts kissing her. Things escalate and he ends up getting her off with his hands right there in the interrogation room.

Bonus points for emotionally manipulative things being said while he does it, like asking her if she hates him now, if she loves him, etc.