I had the same question, but they told me that you act naughty with Anna from the beginning and then when you have sex with her for the first time, you continue to be naughty to free her slutty route.
If you don't act naughty with her, or choose "pure" options you get the lover route, she'll be more loving that naughty, what I can remember different is in the park and convention scenes, plus how she's dressed in the kitchen
