Why felix had knowingly sex with his father

I was also thinking about it and below are two theories about it. Take your pick!

First Felix just wanted to get to know his father Johan and hence frequented the pub. But during the scene where he embraced Johan, he realised that he has sexual feelings towards Johan (maybe not as his father) and so he runs away abruptly. Now he is torn between revealing his identity to his father and having sex with him without revealing his identity. He obviously succumbs to the latter but starts feeling guilty immediately. Hence he throws up after sex and also hits his father later.

Another possibility is that Felix knew that his father was gay and hence felt that the only way he can now get close to his father was by having sex with him. But in the last scene (his father playing with the girl), he realises that he was wrong and that his father has parental feelings though he is gay. So he goes away without revealing his identity.

So much to see, so little time!