If your sensitivity is at max, I think you can't raise your shame.
Also, you should get the ball and not return it, you'll just keep it forever unless you talk to the father without the pantsless pajama. If you want to talk to him without the pantless pajama (you get it by studying until 12:00 PM and masturbating), you need to do it in the morning with the ball you got from the kid the previous day.
To get masturbation, I'm not entirely sure if this triggers it, although it should be the most logical thing. You need to go in the park at night and go to the character's right after the initial park screen. There you'll find bottles, a tent, etc, examine these in the day if you didn't get any type of event at night. Also try to examine on the other side (character left, then down instead of in front, through the fence), and examine the fireplace (top of the fireplace to get a text about "someone living here"). The event at night should feature a homeless man that teaches you how to masturbate... VIOLENTLY.
But man, I STILL DON'T KNOW how to get max curiosity. I'm at 6, I need 7 to continue my events. I can't understand the Japanese with Google translate.