See the part I am mostly pissed about isn't the sleeping with the 17-year-old (fucked up but does happen in this setting and they are Dornish, they fuck anything that moves. Still seething mind you.) it's the attempt at accusing her of a crime even after given reasonable doubt in regards to what could have happened. This, to me, screams like an attempted cover-up. Maybe I don't know the whole story. Maybe Lord Fowler is blackmailing him through this scandal and has him by the balls and he sees a way out. Maybe he thinks that this was some Yandere thing and is trying to save himself. All I know is that he doesn't come out smelling like roses in this situation.