If I'm not mistaken, you can have slept with her before and still get that scene.
What you need is:
1) To reach a minimum number of love points with her (4?) regardless of her lust (she can have 0 lust points and you still get that scene; I did).
2) You mustn't knock on her door (if you aren't spellbound the option to knock doesn't even show up) but break into the house instead.
It's easier to get that scene if you keep it professional, always respect her, etc. You will arrive at her apartment with 6 love points that way.
Another way is to avoid/stop being spellbound and also take Victoria to the party in Ch. 2 because by doing so you can get a couple more points with her in Ch 1 and 2. This way you can have sex with her and still reach the minimum love score.
Have I played this game too many times?
Maybe? Perhaps?
Who knows?