After first date night (visiting Rebecca at the bar), DO NOT accept Abigail's offer to sleep in the bed with you. This will lead to a game over.
After the second date night (at the club with Abigail, Meiko, and Lilith), you'll go to bed with Abigail, and this time, she dresses down far enough that Frank/MC will demand that she write this down so she doesn't freak out. She will initially freak out when you wake up, but Frank/MC will automatically hand her the note she wrote, and she'll be fine with it after that.
The second date night happens the next Friday, a few days after Frank/MC tells Abigail the real truth about their family relationship (I'll avoid spoiling it for those who haven't played through the story yet).
Best case choices are also highlighted in the
official walkthrough.