You are right, I quickly checked and it seems the first time that a "pure lesbian" event has also filthy points. It happened at the club with Patricia and then Tyler.Just realised something. The Nathalie event has a GW/Filthy choice, with the ideal choice being the Filthy one. So we might be wrong about Nat being the woman for the GW Threesome.
On the other hand you have the Antonio event that is in the lesbian route with Alyssa but that only award filthy points.
He once explained that "Filthy points are more like cheating points and finding other ways "to get some fun"!":
So I don't think that the filthy points awarded here automatically place Nathalie in the FW. She played a game with Sophia to challenge/push her and the filthy point is for Sophia being "naughty" (and for whatever will happen with the postman in the future, if anything).1) For the Good Wife path it was actually planned that Sophia will experiment a few things with her husband. With the oral sex scene and Sophia's reactions on mouth-creampie and deepthroat, I tried to imply, that her behavior to this could change later! I mean, why shouldn't it? How do you come to the conclusion that this won't happen? You don't need the filthy points for that because Sophia will do that as a Good Wife! Filthy points are more like cheating points and finding other ways "to get some fun"! Sophia can also get filthy in love for her hubby!
That does not mean that Nathalie will be part of the threesome in the GW either. We have hints from what Liam said but still no clear evidence obviously.