He could not resist the prospect of his daughter being fully grown and stark naked, I mean when he saw her daughter lying naked, he fixated his eyes on her, didnt want to leave despite his wife repeatedly asking him. and this cannot be a shock or he wont be leaving so calmly finally...
I agree with you. I think the father was way more bothered by the incestuousness of his children sleeping naked together with Matthew in a way that appeared post-coital. He was stunned and wanted to do something. The mother, in my opinion, seemed less bothered - almost as if she always suspected something about her children. I thought that scene was indicating that the mother had a sense of intuition while the father was completely blindsided. The father seemed a bit dim, like one of those intellectuals well versed and knowledgeable about poems, literature and history but kind of clueless about what was occurring right under his nose.
Theo and Isabelle share that same kind of cluelessness with regard to the growing revolution and rioting going on right outside of their apartment in the Parisian streets. Theo had some clue, as he met with one of the protester's briefly and kind of blew him off. Even his intellectual Maoism took a backseat to the sexual Shangri-la he was living with his twin and their "boyfriend" Matthew.
One of the droller bits in the film was when the father, having seen the three naked students in the tent, picks up an empty wine bottle, looks at the label, registers that those delinquent bastards have polished off his Chateau Lafite '55, and appears to be rather more distressed by this than by any sexual shenanigans they've got up to.
"I beseech ye in the bowels of Christ, think that ye may be mistaken."