You can basically play around the scenes that cause the game to detect that desire goes down. When the girls ask you to have sex you can deny them without consequences, if you avoid peeking through the hole they won't masturbate at school, and if you end the day any way other than sleeping (for example, having a late night sexual encounter) you won't get the video where they masturbate at home. This way you can effectively hold all girls at max desire indefinitely