It makes a certain amount of sense for Lanzhu, actually — you could spin her as having a pretty sheltered upbringing (compare to Maki, who still believes in Santa), and she's definitely had exactly one real friend growing up, from whom she's often been separated. You can fairly easily construct a view of Lanzhu where her isolation has left her very innocent and ignorant.
That said, it is a bit exploitative to launch into sex with someone who didn't know about it previously. Informed consent, 'kay?