You took the wrong last step there. You should've chose Charlotte's Route when she asked about something between you and Anna on the Yacht on Monday.
Charlotte is Anna's best friend so she won't take anything that belongs to her ( Anna Route ). But if you say nothing happen between you and Anna, then she wants to have you for herself ( Charlotte Route ).
That's why it's called Harem End since Charlotte and Anna will argue, wanting to have you for each of themselves and you decide to have them both instead.