When it reach 9/9 an event happens, the girl shows at your door and ask for release, you can turn them down twice and if you do them an unavoidable event will take place (Link breaks stuff, Kali drugs you, Ashley ties you up(?, Android brings friends to play, etc).
Adding on to what EmiNVS said, lust is gained along side sluttiness when a character has the Mega Slut trait and is above a certain relationship level.
