So the scene is specifically related to her "rejection" path. Essentially, the MC resists her advances up until Chapter 5, when she comes into your bedroom at night... before finally giving in and accepting her advances. They're then notified that if they reject her again, her path will be closed permanently. It is here that players need to accept her advances/let her stay. This makes the MC react harshly when she gets cold feet about sex in Chapter 6... as he feels like she's giving him mixed signals and being childish.
So the prerequisites to unlocking the scene are as follows:
1. Reject/do not pursue her romantically until Chapter 5. If you do this right, you'll get a scene by the lake where she pulls you aside to speak to you about "us."
2. Doing so should prevent her "horny" or "love" points from being too high, but just in case, make sure you make choices that keep these values low... such as treating her poorly or harshly, not flirting too much, etc.
3. In Chapter 5, let her stay and let the lewd scene play out.
If you do all of this right, you'll get the scene (and yes, patch required). If you did all of this and you still do not get the scene, avoid saying "I love you too" in Ch6. Instead, say "I know" or "I care about you"