Why are the Japanese so darn comfortable with their sexuality?

I first became vaguely aware of this phenomenon as a mere youth devouring any anime I could get my hands on. My first experience with hentai was, like most, the OVAs Urotsukidoji. With child-like characters being penetrated by demon penises (sometimes by characters with multiple tentacle like phalluses who could bring a lot of, erm, [...]