At this point 80% of my comments here must be about Sage. But what can I say, I love the character.
To me, Sage's relationship progresses in a completely different way than the other LI's routes. While the others needed to first establish an emotional connection before getting in bed with MC (this is especially true for Bella and Jill), Sage is the complete opposite.
The way I see it, the point of her relationship is getting to know the real Sage, romantically and sexually. We already know that she loves sex and wants to experiment a lot of new stuff with MC, but that's because she's a lot more open to talk about sex than anything else.
The real "challenge" of her route is to bring out that sweet and caring Sage that we see in Season 2. Sure, she was an emotional mess in that season, but that's when she also realized that the MC cares about her a lot more than she thought.
During those episodes, we see that she has her guard down when it comes to emotions, and that's why we bonded (emotionally) so much with her. That, to me, is the point of her character arc: to show Sage that it's okay to let your guard down to the ones you care about. And when she finally realizes that, the "romantic" Sage will be a lot more common to see.
I agree with the fact that she's not as emotionally involved with us as the other LI's are (she likes us a lot, don't get me wrong, but she's not IN LOVE yet) but that will happen with time. If the whole point of her relationship was just to try sex stuff, then we would have no challenge or goals as a couple because, like she says:
If sex was really all that is to her, why would DPC make her a main girl in the first place? She could just be a side girl and nothing would be different. Or, narrative wise, she could just refuse MC's proposition and tell him that she only wants to be a fuckbuddy.
Of course, her route will be a lot kinkier than Jill's and Bella's combined, but that doesn't mean it will be just that.
I have no problem with how Sage was portrayed in this chapter, it's completely in character with her. They are in a party and she's finally free to try out new stuff in bed that she didn't got to do before, and what we see in episode is her finally letting loose after all these years. But I would also like to see, in the next episode, how she will behave with us in private when the environment is appropriate for a more serious approach to their relationship.
The sad truth is that we will have to wait for 7-9 more months in order to see how things will progress.