The stuff with Mark can be prevented as soon as he is introduced in chapter 7. Partygirl shows wallflower a picture and MC can either encourage or disencourage her to flirt with Mark. You chose to let them flirt so wallflower hangs out with Mark in chapter 10 by partygirls schemes.
Ehh...
First opportunity is to fuck Cherry. A bit later, Alison asks for another date where she can be fucked. Ian can date Lena step by step and finally fuck her. When going out dancing, he can go for Emma. Alison is pissed at this, but it frees up fucking Cherry again later. Then Ian can have an affair with his boss, get in a open relationship with Lena and fuck Cindy in secret. In chapter 10, there is a threesome with Alison. Instead of Cindy, he can also screw Holly, but Lena and Minerva don't like that.