Long, long before. The masturbation scene is fairly early on in the game, before you and Rachel go to the movie.
If Rachel has high enough corruption (5) you can discuss the movie more in-depth. If agree
with Rachel and find the movie arousing she'll earn trust and corruption. (Trust
+1) (Corruption
+1) Then you'll have the opportunity to start your relationship with Rachel by choosing "Would
you like that?". Being modest about your sexual experience will gain you trust.