The Five Stages of Zerophilia
1) You're born however you're born.
2) You lose your virginity, and start to switch whenever you're aroused.
3) You have an orgasm as each sex (signified by Luke/Luka having problems, while his friends waited outside the door).
4) You can change at will. We see Luke and Luka switch several times without seeing them having problems getting excited enough to have an orgasm each time. Also, Max didn't have enough time to... get things going while Luke played with the drill. And, again, it didn't seem like Michelle was pleasuring herself in the bathroom, it just sort of happened.
5) You have sex with a Z in order to lock in your gender, until you have sex with a Z again.