Sorry to drop in like this - he's not guilty or defensive, but on the other hand he's genuinely angry and hurt that Jack doesn't care for him more than he did ('When did you last ask me anything about my life'), and uses this to put Jack on the defensive, which surprisingly even works, because that's when Jack calms down and puts away his gun. I think in his mind he justified sleeping with Jack by telling himself that he was doing this to protect Lisa, and justified betraying Jack (and I think he saw it less of a betrayal, because he was so absolutely convinced that he was doing the right thing) by believing, rightly or wrongly, that he didn't mean anything to Jack.