"Then add soy sauce to the butter..." This is the sign for me to drop the manga. I can tolerate stupid filler plots but soy sauce in brown butter sauce is way too much.
What she did was surprisingly science-based. She forced his body to release its dopamine (by ejaculated) so his body would not feel the effect of adrenaline (from the feel that he accomplished something). And then the only things left in his body are oxytocin (from the affection toward her) and...