FERA-124 Nearly incestuous sex with mother, only allowing penetration of the tip

Yumiko had been tormented by her son's sexual harassment. On the day she was caught masturbating and accepted erotic acts as hush money, her son started demanding various plays, with the content escalating more and more. Eventually, she was cornered to the point of being about to be penetrated. Yumiko desperately resisted, saying 'That much is no good,' but her son wouldn't stop. Finally, he proposed a forceful special rule: 'If it's just the tip, it doesn't count as having done it'...