It's an old Japanese game and they have strict laws and rules regarding media for porn.
The acts themselves are allowed, but the visual representation of private parts are not. Which is why censoring.
Fun fact, early attempts to bypass these laws resulted in the start of the furry and kemonomimi culture. Since if you stuck animal ears on a girl it wasn't technically a human or a representation of reality and could be uncensored. That loophole got plugged real quick though.
As an aside, I faintly recall this one. Super old game. Actually had an official english release which got bad press for the subject matter so the Japanese version was in circulation more. Since no one in the States cared if they couldn't read the moonrunes.
