Just read what Lien tells you. And yes it is. Cause the Quest is: Go as far as you wish to go, and feel comfortable. So indeed the Quest is finished right after Max entered the Room. Lin tells you 10 Times that you can have Female, Male or Both clients. This Quest is only optional. So if you want to earn Money there, and don´t want to do lewd Stuff to Men. Then just End right at the Point you want to go.
The Quest IS already ended before the Last Scenes start. Yes it is quite missunderstandable. But I asked the Same question on Patreon and I got exactly this Answer. The Quest is Finished with the Last Advices from Lien.
Patreon quote:
Me: ... Is there a way that Kate only Massage Girls? Cause Lien metioned it, but I have no choice but to Fuck with Max. Would really appretiate this. Or even change the Talk Lines with Lien that it is clear that there will be no Going back. Similar to the Sex with Nick. ...
Answer: 4 You do have a choice!, to not fuck max look to the bottom LEFT, there is a "Finish" button there.
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So the FACT is that you decide when, and how to FINISH this Quest. And yes it is quite irritating, but only the Devs knows why he makes it so confusing.
See it as the Question same Games ask. "I like to massage Women", "I like to massage Men", "I like to massage both". But instead of just questioning, this Games takes you through some Scenes, and you end at the Point you want to.