There are a number of different combinations for this. Simply vary your answers so that you are not being faithful to one girl in particular. One combination that will get you the bad ending is this:
Who will you go with?
- Go with Ageha
Do you like Amane?
- Tell Amane you are not interested in her.
Who's swimsuit do you like best?
- Say that Kotori's or Amane's swimsuit is best. You will only be able to choose Amane as an option if Kotori's route has been cleared once already.
Who's turn is it tonight?
- Ageha's.
Who would you like to go with?
- Go home with Asa.